Hmm, personally I like them all but I am leaning towards Firemage as the most satisfying.
There is a very significant problem though with the balance and difficulty of NOTR making the early game pretty difficult for Firemage and making it relatively easy to gimp a build if you don't focus it correctly (Do a bit of warrior on the start to get you by up until 1st lvl of 1-handed sword i.e and save up LPs for when you get accepted etc.). I like fooling around on the Gothics playing hunter and even smith early on (and definitely getting a bit of fighter in there too) so I almost gimped my first run through it and had to go to some pretty silly extremes to pass a few hurdles before getting it back on track
I believe the most accessible and straightforward build is the Paladin and its pretty satisfying to play. Twohanders and crossbows make for a good Tank combination. Story wise the Dragonslayers might be just a mite more fun and you can craft some pretty cool weapons too. Hmm its damn hard to choose between those two. I completed the game with all three factions and would not have it any other way