It's reflective in price which is the best. The 550D as mentioned will only really give you extra megapixels (which to me are actually fairly important). Other than that, and the gimmick of HD
recording, it's basically still a 350D.
The 60D is much more than a 550D, contrary to popular belief. Yes, it's plastic, but it performs more like an XXD model than an XXXD. It's faster than the 550D with slightly better metering and focussing systems.
Then of course there's the 7D, which is essentially a pro camera. Awesome metering and focussing, dual Digic IV
processors and lightning fast. There have however been noise issues with it, which I don't think have been fully resolved. Still, it's without doubt the best of the three.
I think you'd be best off with the 60D (7D sounds overkill for your needs I think) - although the 550 is now the 350's fourth-generation successor. Go play with them and see which best suits. May I also ask what lenses you currently own? This could be a factor, which I'll explain once you've given an answer.