There are so many different rigs it is almost impossible to say x,y or z is the best... Personally, I use a Razor, but have also used the X-Deep (tec wing) on several occasions, and it's a very solid, well built rig. Either one of them will handle aluminum tanks or steels. On my razor I have put up to 6 tanks at once and it was able to handle it perfectly. The majority of my students who come with their own rigs will do so with a Razor or an X-Deep. On Occasion I've had students with Apeks rigs (another solid piece of gear), but those who come with Hollis SMSwhatever or Nomads. we usually end up having to modify the unit so much, that by the time it is set up properly, we could have set up 5 other ones.
Whatever you end up getting (I suggest you talk to your instructor prior to), make sure it is a dedicated SM rig that you feel comfortable using and that is appropriate for the type of diving you will be doing so that you only spend the money once... Those that claim to be good for everything (single bm, doubles bm, sidemount, etc) end up sucking at everything.
