It depends a bit on the specific conditions you are to sail in.
In epic, flatwater (with the mandatory rolling chop) the asymmetrical fins from Time Machine (TM45v7) and KA/Lockwood (20cm and 21.5cm cm) are perfect in my experience if you have a starboard tack course. The TM fins have the edge in tight conditions and the Lockwood fins are sometimes a bit easier to get to perform in very disturbed or rough water. Bigger guys would like the 23 KA/lockwood in Assy as well.
For less than record breaking winds and/or a tight course and for more versatility on both tacks the Time Machine 26 Speed is sensational and there is a symmetrical version of the KA/Lockwood that is very nice if you need a swept fin to help shed the occasional bit of light weed.
For symmetrical fins the Techtonics has a good reputation but I would caution against going smaller than about 25cm unless it is ideal conditions. I have personally not had much success with the smallest Select speed fins (21cm) but the larger ones over 25 seem fine and some of their slalom fins are excellent.
In some conditions a swept back (or set back) fin also seems to help get the boards nose up a bit and feeling looser and Foxy had had great success with the Swept back C3 fins
The older I get, the better I was.