I've experimented with all of the mage and rogue specs on my Inquisitor, but only one warrior one. I HAVE played all of them on companions though.
For Mage, I think KE is the easy button. You cannot die if you build it correctly, and it's got the sick-ass anime sword animation. It's sweet as hell to shatter a frozen enemy or the like with a giant, one-handed slice with the fucking lightsaber you get. But in terms of fun? I think Rift Mage is fun, because it's not infallible, but you really feel like it's your effort blasting everything to bits. Necromancer is nice, but for some reason, I can't ever seem to deactivate the spell that reanimates the bad guys, so I have to wait for the spell to run out before combat ends, and that gets old.
For Rogue, be a Tempest or go home, tbh. I like the other two specs for their flash, but the friggin elixers you get as a Tempest make the game SO exciting. Need to disrupt a rift/revive a buddy in the middle of combat? Use lightning and fucking stop time! Swarmed by baddies and all your escape skills are on CD? Use the Ice one and freeze them all. Want to do that cool burst-fire flip three times in a row? Cover yourself in fire and flip away! The fire one can even let you use Focus abilities for free, but I'm pretty sure that's a glitch. Assassin and the other one (Tinkerer I think?) are fun (especially the thing where you just throw like 20 traps out onto the field), but they're pretty useless comparatively, and really only add more of the same. With Tempest, I was able to kill the LVL20 Hissing Wastes dragon at level 12, and I took hardly any damage while doing so. Add Sera to your group, and then you have TWO of these beasts. With Tempest, Nightmare felt like Easy.
And I suppose, for warrior, it depends on what you like to do. Do you like to tank? Champions cannot die. Do you like to buff allies? Go Templar. Want to absolutely wreck everything in your path, so long as you're good at management? Reaver'll do that for you.