I can only speak as a customer, but so far I've had the best luck with Virtuozzo. I signed up for a VPS from zone.net last week for some testing, and so far the performance has been great. This is for a Linux box, not Windows.

I'm also eager to try out a decent Xen vps, but the price is always at least 25% more than a comparably spec'ed Virtuozzo VPS. It's just not in my budget right now, but hopefully will be soon.