Fins are one of those things that are incredibly subjective, both as to performance and to 'best' in terms of quality. Moreover, your question is open to interpretation. Generally speaking, fins used for spearfishing which is done while freediving - e.g. while the 'diver' is snorkeling - tend to be very long. Fins used for 'spearfishing' while on scuba are generally shorter, fins designed for scuba diving - not freediving.
On the homepage header, just below where it says "welcome Hardwick" you'll see some function buttons, third from the right is the "Search" button. If you put 'split fins' in there, you'll be able to search through many threads in the archives where fins are argued over, debated, etc.
Like many other things in life, fins fall on a continuum. Split fin users maintain that they are easier to use, offer less resistance to your leg muscles, hence lessen likelihood of cramps, etc. Perhaps they do - but they tend to move less water, so in current your performance may be compromised. Of course, if you have powerful legs, you don't care. Other users note that the kick cycle for splits may vary. There are observations on both sides of the debate.
Best advice is to get a pair that fit you, and rent different pairs until you find a fin type that makes the best sense to you, for the type of diving you intend to do. You'll get all sorts of input on the boards, some of which may apply to you, some of which won't.
Some searching will offer you a wide variety of opinions to explore. Good luck.