[Faceman]

Assassin: Always interesting, definitely worth playing if you have never had a PC thief. As there's a lack of NPC thiefs in the game but there are lots of nice thief-items around this one is the perfect completion of the party. Also with the right proficiencies (Long Sword, Katana) the Assassin can be extremely deadly in backstabbing (septuple!).

Priest of Talos: Interesting for an evil party but if you've played the game sometimes already I assume you won't find a lot new in handling a cleric unless you never used Viconia or Anomen. Also evil clerics only get to charm undead which can be really annoying while good clerics can just destroy them.

Mage/Thief: I assume you re talking Multi-class here. This is basically Jan Jansen. Not a bad choice to play but not really interesting. Nothing new.

Fighter/Druid: Can be really interesting because Druid and Bard are the classes most seldomly chosen. Therefore many people do never get to see their strongholds. Just make sure you don't take the keep if you decide for this one.

Cleric/Mage: Nice choice if you want to improve your magical knowledge. A cleric/mage is great to try out the combinations of clerical buffs and magical attacks.

So after all I'd suggest the Assassin if you've never used a thief as PC he is a must.
If you've always played thieves and fighter-types I'd suggest you get your proper magic dosis form the cleric/mage.
And if you're in for something less normal go with the fighter/druid.