For me, I'd say the following:
1: WAR. Just straight up feels like a badass tearing through everything. You are the beast.
2: PLD. "Oh my! The wall is impregnable!" (quote taken from Beyblade for the Nintendo GameCube) Seriously. Whenever I play my PLD I feel like a titanium wall for the party.
3: AST. It's all in the cards. The RNG factor of the cards makes this class constantly keep me on my toes.
4: BRD. This class makes me feel like a pillar of support.
5: NIN. Bish bash bosh BOOM! Seriously, thanks to the improvements made with the new EU datacentre and the changes made in 3.1, bashing out a mudra is such a satisfying feeling now.

Honourable mentions:
The DoL: Very relaxing and easygoing classes, great for those times where you just wanna chill out.
The DoH: I feel like I'm doing mathematical problem solving playing these classes, like how to best tackle the problem I am set.
MCH: Same as BRD, but because it's got a lot less support into it, it just misses out.