I like the hectic playstyle of DRG/MNK, but BLM is hilarious coz you can stay at a safe distance and just put shit on blast. I have bard at 50 as well, but It doesn't seem as much fun as other stuff that hits harder. Also, you have to focus on stuff other than pure DPS. Screw added responsibility, I just wanna hit shit.

You immediately contradict your thread title when you go from "most useful" to "most joyful to play", then again switching to "best" (what does this mean?); I really don't know what you're looking for here, so I'll give my opinion on several "bests".

Most useful: Probably bard, healers like the security of having a bard around; but I believe in balance, the faster shit dies (with a higher dps class) the less heals/tp you need /shrug. I say "useful" based on general community opinion
Most fun: I think melee is the most rewarding, and therein fun, because you simply have to deal with crazier mechanical issues (dodging, flank & rear attacks), BUT BLM can be so bursty with procs, I can't help but laugh when I play it. This is a tie =p
Best: This one is pretty black and white, MNK/SMN do the most damage, and in that respect are the "best" DPS (best deliverers of damage-per-second)