Nearly impossible to answer.
Browns offer the most challenge on a daily basis, bows are more fun to hook but smallies do probably offer the most consistent fun. I really don't know.
Depends to some degree on how much solitude I can get in the process. If I can be relatively unbothered or crowded, wild brown trout. Otherwise, flowing water smallmouth which I can always do with a lot of solitude.
I'll add this after reading some of the answers above. I didn't consider tarpon and bonefish...I love fishing for both. The problem is, if either were my answer as the one fish I can only FF for, unless I won the lottery and/or moved to a Caribbean Island or someplace like Belize, I really wouldn't be able to do much fishing.
Smallies in and around pee-aye where I live is still my answer.