I'm assuming you're playing without ToB or any Mods except the Bonus Merchants. Regardless, play what you like (of course).
Virtually any class can work nicely in that party, but an obvious choice is a pure damage dealer - Sorceror, Kensai, Monk, Assassin, Archer or such like. If you go Kensai you've got a huge choice of good weapons to pick from, Sorceror should get SotM, I'd think. Assassin - probably longswords or staves +ranged. I believe Antipaladins are in
Tactics MOD amongst others.
On the Paladin kit side, I'd pick Undead Hunter over Cavalier - the immunity to level drain is damn nice, and significantly more useful than the Dragon/Demon bonuses.
For your illusionist thief, I'd start with short bow & either longsword, staves or short sword. Also consider Staves or Spears (Impaler, yum!) for your F/D. Viconia will mostly want to use Mauler's Arm to get her strength up to a decent level, but it's available to you pretty early on.
Obviously MODs installed may significantly modify your weapon choices.
Good luck!