Assassin isn't as good as straight-up Beguiler (spells are the awesome), but it's legal (sneak attack is a perquisite of Assassin, not a prerequisite)... and the level of sneak attack and death attack it gives doesn't exactly suck.

So if you want to be useful in combat, you can feint as a free action then hit for sneak attack damage. But did you spend two feats for sleeve blade and longbow? If so, why?
Ok, so the case can be made for longbow. Distance between you and your enemy is good protection... but it might not be worth the feat over shortbow. But if you use longbow, don't waste the feats for Improved feint.

If you do want to feint in melee, rapier is just fine. Or if you want a weapon feat: Greataxe. The greataxe does lots of damage (and adds the feint's sneak attack die just fine), and also helps prevent enemies from selecting you as a "juicy target". 2-handing is a fine option for arcane casters since you won't be wanting a shield anyway.

In short: I wouldn't waste too many feats in combat. But if you do, pick either the longbow or a melee weapon such as greataxe.