The entry level 270EX should suit it fine, if that's what you're after..
I've been using a Jessops 300D with my 5D Mk2, but it's a bit of a pig if you're not used to doing your settings manually. For this reason, my next purchase will be a flash that can sync properly with the camera - I've spent this much so far, so I'll probably go for a 580EX II once I've saved up a bit.