Can't pick just one, so... here is a top four!
1. Black Mage.
I used to think they where really cool when I played the first game, but then when I was a teen I went through this whole "I hate magic users" phase for whatever reason. Either way Black Mage was always a cool little guy, very iconic sprite from the first game and of course just an engine of destruction. Playing as a Black Mage in Final Fantasy XIV has been amazing and a lot of fun. I just love destroying things with magic it seems.
2. Dragoon
I like polearm weapons, their armor looks awesome, and the jump ability I always thought was cool even if you did have to wait on it. A team of all Dragoons in Final Fantasy Tactics made me giggle.
3. Gambler/Corsair/Lady Luck
Does this count? It doesn't show up as often as other jobs in the game, but I love the whole gambling aspect. Using cards as weapon, relying on the roll of the dice. Setzer was my favorite character from Final Fantasy VI and it's actually what got me into playing poker and such. Corsair was also my main job on FFXI, it didn't throw cards, but I got to shoot elemental ones with my gun as well as buff people with dice rolls. Hitting that XI and getting the best buff for the party was so sweet, but busting was a bummer.
4. Gunner
Not really an official job (or at least I don't think it was?) until Final Fantasy X-2, i just like well shooting things sometimes. I suppose Vincent and Barret would count since their weapons are guns, but only Barret had limit breaks that really shot anything. Irvine in Final Fantasy 8 had more gun play with his due to all the different ammo types. Still just because it's a fantasy game I like that they've introduced this job and the idea of just shooting the crap out of your opponent. I guess this one has a bit more character force behind it for me, and maybe Corsair could fit here too, but yeah my other favorite way to blow up faces is with a good new fashioned gun. ;D