Whatever fin best allows you to swim energetically for a sustained period.
Do a thought experiment: Imagine you are in equilibrium in a large body of water. With your eyes closed, can you determine which way the current is moving, or even if there is one? Hopefully your answer is "no". Barring local turbulence, you are part of that body of water, moving with it. If this is hard to understand, it is similar to riding on a plane or train. While you are cruising, it feels as if you are sitting still (except for vibration). It's no more work to walk forward than toward the rear.
So why not try several fins in your local pool and see which works best for you in still water? Physics suggests they will also be the ones that work best for you in current.
Scratching my head with my flippers on in the back of an enclosed truck doing 50mph, if I jump in the air, how fast am I finning? I use half a dozen different fins, covering the spectrum, a few different kicks, crippled frog mostly, and other than propelling me to where I'm propelling to be blowed if I can catalogue the propulsion technique that propelled me to where I was propelling to, on a dive, or subsequently. Never underestimate the importance of cadence.