It's not an easy question to answer, especially if you change your party again when encountering other potential members. Every party member has their strengths and weaknesses. But given the choice, I'd suggest Jan, because he's more varied than Viconia, and you've already got a reasonably good cleric. Jan is a fine thief and a decent mage, while his blaster bolts (which no one else can duplicate) will actually stun even the worst opponents after you lower their magical resistance. In one of my run-throughs, Jan actually stunned the large demon in the Planar Sphere quest. Even if you don't know what that is yet, trust me: it was damned impressive.
They also make an interesting combo in parties together.
And finally, bear in mind that party members of very different alignments tend not to see eye-to-eye.