Tielle is my go-to adventurer now, mostly because I tend to always clear as much of the dungeon floor as I can, at least until the Wisps chase me out. It's a habit. Tielle is awesome at clearing rooms, so I'm happy. For everything else, there's Cuterage, which I suspect is overpowered.
I hear Griff might be better, but after learning to stay at range and blast away blithely at enemies before they hit me, Griff suddenly feels like I need to relearn all my melee timings.
Caillou is an odd case: I can see how he can be awesome, and I agree that against bosses he might be second-to-none (haven't tried him against humanoid bosses yet, but I am impressed by his performance against non-humanoids), but he's made of wet tissue and I end up worrying about his SP usage more than I think I should. It's probably a problem with the way I think, trained from old Final Fantasy games: conserve SP (or MP or whatever it's called) until absolutely necessary. So when I use Caillou's SP, I end up looking for regular critters to use my melee or no-SP fireball attack on, just to get it back up to full.
I can't play Charme or Elan effectively, mostly because of their dashes. I always end up accidentally triggering it somehow and then I crash into a wall and I'm vulnerable against critters. I suppose they have their strengths, but they just don't do anything for me.
Louie is, as usual, the old standby. Hard as a rock and about as smart. I love watching his scenes, and he's got the standard Zelda-esque wide arc.
I haven't tried Nagi yet. Maybe after I finish the Lapis Ruins and start working on levelling everyone else.