Spellthief, correctly built, can be amazingly potent. We had one in our Age of Worms campaign, and it was just wrong. So very versatile - and on anything that it could sneak attack (which included undead and constructs for him), he would strip it of all its significant abilities in no time flat. For the 'rogue' slot, I've got to go with the ninja. It's just bad. Compared to just about any other candidate for the slot, it's quite inferior - nowhere near as good as rogue, spellthief or scout.
For the 'arcane' slot, I'd go with sorcerer, except that it's core. Let's go with hexblade instead. It's bad as a fighter - it's even worse as the primary caster (it's great, however, as a base upon which to build to certain pclasses). You'll get a durable arcanist, I guess, but only level 6 spells, and pretty limited spell selection on top of it.
For the 'fighter' and 'cleric' I'll go with samurai and shugenja, for reasons others have outlined above.