One suit for all jobs? Please...
Would use what's most appropriate to the situation. However, I'm a great fan of sniper rifles (and sniping as an assassination method) so I'd likely have an urban ghillie (or several of them actually in different colours for different kinds of buildings) suit that'd be my go-to thing for most stuff. Other than that, business suit, lots of casual clothes most befitting the situation etc. Would aim to look like someone completely ordinary if I'm doing a close range kill.
I'd love to have one suit specifically designed to be able to soak up some blood (without it being noticeable obviously). I'm sure it can be done with the right material and it would be brilliant for close range kills with an Assassin's Creed-style blade. If it couldn't be done, I'd go with a suit that has several layers that could simply overlay a bloody sleeve. As far as the looks on that one, I'd go with a business looking suit cause that's always wearable.
edit:
Oh. And a black, long trench coat coated with armor (Kevlar or something of the like). For them... unpredictable situations
