There were lots of good points brought up so I'm not sure I'm going to add much value here but I'm happy to give some input. I own a rEvo, SF2, and most recently a Defender which is the cousin to the X-CCR (Hammerhead platform).
I can't unfortunately speak on any of the KISS rebreathers but they are popular units. The biggest complaint I hear is build quality and flood tolerance. Unfortunately there doesn't seem to really be a good alternative to the Sidewinder so people seem to just deal with it's flaws. Personally I don't think a sidemount CCR is the best first rebreather. I have friends that dive KISS classics and we've done lots of dives together but I just think there are better units on the market.
I originally started on a rEvo mCCR and put about ~850 hours on it. It was my first rebreather and we had a lot of fun adventures together. I dove the sh*t out of it. I still like my rEvo but for me the Achilles' heel was lack of flood tolerance. For this reason alone I don't feel it's the best unit for cave diving although others would certainly disagree. There are some good features on the rEvo such as dual scrubbers, smaller profile and ease of assembly. I'm also a fan of the fully redundant electronics and 5-6 O2 sensors options but some people find this "gimmicky."
The other thing that really annoyed me was rEvo's "Apple" like philosophy of not wanting users to open the case. It was the policy of rEvo USA to not sell counterlungs to the end user. Granted this is not a simple install for the average user but I firmly believe users should be allowed to service their own gear. My counterlungs in my rEvo are a combination of tear-aid/aquaseal holding them together but they don't leak anymore.
My unit was also old (original rEvo II) and had 2 previous owners prior to me so it had a lot of mileage.
With the exception of the counterlungs / internal bulkhead gas fittings the rEvo is extremely self serviceable. None of the electronics are potted. There is really no "head."
There was some uncertainly about rEvo support when Mares took over. I can't speak for last couple years but it seems like parts and servicing is not an issue. It WAS an issue when Ocean Edge handled rEvo servicing. I think ultimately maybe the Mares distribution is a good thing. I lost exactly 1 dive on my rEvo due to O2 leak. I never had to bailout and I never fully flooded the unit although I did get a lot of water in it once when I was brand new.
There is a lot of rEvo hate/drama on Scubaboard. Mainly I think because of the AddHelium/Peter Sotis debacle and because of a few high profile cave instructors in Florida who simply hate the rEvo and people like to follow the herd mentality there. It's really popular to hate the rEvo and joke about it's 11 ridiculous oxygen sensors. Some of it is deserved; there are some definite improvements that could be made to the aging design.
After the rEvo I bought an SF2 and put ~350 hours on it. I wanted to go full eCCR and I wanted a unit that had good flood tolerance for longer duration cave diving. Personally I think the SF2 is one of the easiest units on the market to de-water. The SF2 is streamlined and less "busy" than some other units. By that I mean everything is housed in the carbon fiber body.
My biggest issue with the SF2 is work of breathing and uncertainly about future USA support. ScubaForce USA has changed management/ownership about 5 times in 6 years. There is currently no way to get your head serviced in the US and all the electronics are potted inside.
The unit breathes great in trim but not when you're in an extreme heads up position (waiting on surface for a boat) or going completely head down through a restriction. I don't believe the SF2 travels well. It breaks down into smaller pieces but the latches on the canister body really prevent it from breaking down any smaller.
One thing the SF2 has going for it is that it's incredibly easy to assemble and a very simple design but there are some pieces I don't like. The head is a pain in the ass to get off without their special "head removal" tool or standing over it and/or pulling on vulnerable fittings. The SF2 head design is also literally two pieces that are silicone glued together. It's a crappy design and the head is prone to leaks there although they wont admit it.
The unit is also very long/tall compared to other units. As someone who is short the unit is comically long on me. It's not an issue per se but I wish it were shorter. They do make a smaller size lower section (I guess for women with smaller tidal volume) but it decreases the counterlung volume by an additional 1L. I could not get a full breath when I tried it and bottomed out the lung every breath.
I also hate how high the head sits on the unit. It's basically the first thing to hit on a cave / wreck ceiling. It's 100% non-issue but I just have a lot of nice scratches there. You need to train yourself to duck if you're used to doubles or another rebreather.
The SF2 DSV is a piece of crap. It should be replaced immediately with something else. I put a Divesoft DSV on mine.
The SF2 tank brackets are a poor design. They bend and wobble. The welds failed on mine and I had to weld them back together.
I also wish the SF2 came stock with a HUD but this is easily rectified. I ran my unit with two Shearwaters (one controller and one hardwired monitor) because I believe every unit should have two ways to monitor the loop. I did not jump on the Shearwater NERD bandwagon because I personally like two handsets better. I wish it had another monitoring port.
Only one sized scrubber for the SF2. Honestly it's a non-issue but I just figured I would mention it. There are people doing 7-8 hour+ dives on the 5.5lb scrubbers.
I realize I was pretty negative on the SF2 in my post but I still think it's a decent unit. No CCR is without it's faults.
Right now I am brand new on my Defender but so far I really like it. I can't really give a solid opinion of pro/cons until I get more hours. The bayonet head fitting and quick-lock connectors that go to the loop hoses/BOV/head are amazing. I wish these were on every rebreather. I know this is nothing new for Hammerhead users but I really like this design. To be honest-I just happened to pick up a brand new Defender (used but never in the water) for a good price so I jumped on it since I wasn't finding used X-CCRs or JJs in price range I wanted without a significant amount of hours.
All the iQsub parts seem well engineered (I'm sure somebody will laugh at me for saying that.) I love the cable connectors on the head and I really love the removeable cell cartridge.
X-CCR / Defender takes numerous size scrubbers (4.5lbs/5.5lbs/8lbs and even 9lb for the Defender). I'm still struggling to get used to limited head movement due to the T-pieces but I think that is similar for most units like this. For example: JJ, Meg.
I really love the manifolds that attach to the back of the canister for hose management. It makes the routing for the MAVs, BOV, wing, and O2 feed into the head really clean.
The unit breathes well in most positions compared to SF2. De-watering the exhale lung seems straight forward but not quite as easy as the SF2. I'm just not in a great position to judge this yet.
Compared to my rEvo and SF2 I feel the assembly is much more involved process with T-pieces/counterlungs, BOV connection, etc but it's really not bad and I think after another 20 builds or so it will all seem really irrelevant.
My biggest unknown right now on this unit is the proprietary SG1 electronics but I am also running a secondary Shearwater in addition to the HUD.
Some last thoughts - Everybody seems to tout a rebreather as a solution to underwater photography to get "closer" to larger pelagics or not scare the fish but I honestly just don't see it. I've had some pretty amazing encounters on open circuit. I think most of it's just being in the right place at the right time. Personally I think rebreathers are more of a hindrance to photographers who tend to be more focused on their camera and spend less time being attentive to their ppO2 and what their unit is doing. That's not to say it's unsafe but it just requires a lot of attention and task focusing to do both.
I also used to think that I would dive a rebreather every dive and that was true for maybe my first couple years until I built enough hours to feel comfortable/proficient. Honestly for 2.5 hour Ginnie Springs (~100ft) dives I am just happier with open circuit doubles with a stage or two.
There is something to be said about the simplicity of doubles or even sidemount. No disinfecting a loop, no packing a scrubber, no worrying about O2 sensors, dead batteries/handset, stuck solenoid, CO2 breakthrough, flooding a loop, going hypoxic, etc. I'm oversimplifying this but at the end of the day it's much easier to just strap on a set of doubles and go diving. As cliché as it sounds, bring the right tool for job.
I now only really dive a rebreather if I want to do a 2.5-3+ hour dive or a dive that requires a decent amount of trimix (130ft/40m+) or one with a significant amount of decompression. A lot of my dives are ocean wreck dives in the 150-250ft (~45m-76m) range so I dive a rebreather. On the other hand last week I was on a small wreck in 110ft so I dove open circuit
TL;DR - Sorry for wall of text. Everyone has strong opinions about the best rebreather. They all do the same thing. They all suck.
My top three picks (in no particular order) would be X-CCR/Defender, JJ and maybe a Tiburon with BMCLs. Fathom would be runner up 4th but the company is one person and there are only like 60-70 of them in the world.