If your considering someone making 3 turns on a wave a legend, your most likely over thinking your fins or you mean the waves your riding are pretty much shutting down pretty damn quick and your still probably over thinking your fins. Personally I think surface area, stiffness & rake more important...but hey i'm no pro or legend and growing up fins were already on the board so never a consideration unless you lost one.
Different fin types do make a huge difference and there is also a huge difference in waves, peoples weight and surfboard type/size. If your surfing fast breaking pretty much closeout beachies I'd go quads or a large set of thrusters. I like plenty of drive and tend to surf shorter boards in larger waves. I like Shapers S9's thrusters all round for over head & a 1/2 and switch out the rear fin for a solus back small cutaway. Thats my favourite setup for HS Hypto and lovebuzz atm...the solus set on boards with deep concaves goes great in nice peeling waves and AM2 are a good old favourite.
If your under 75kg you can consider medium fins. A lot of younger blokes like KS fins and I'm sure KS enjoys making more money and building more wave pools for us to drool over...personally though, I'd recommend choosing a fin type. Futures or FCS2 and just getting medium, large and quad sets you can mix and match. Boards with sharper rail and or more concave won't need as much fin or fatter/smaller waves.
At the end of the day you will end up with a few fin sets to play with...havagood1