Xen
They all seem to have their own probems. I am switching here from a xen vps which has been very stable. I have had it 3 months and it has been rock solid. The reason I am switching is the other provider has no community support, no control panel and takes forever to answer email. His xen vps's are very stable. I had a 512mb ram, 50 gb hd vps. I have a linode one (or whatever it is..the minimum) and it seems to compare favorably with the xen system. Time will tell if I need more memory or resources on the linode. My configuration takes at least 512 mb ram on a xen system. I have tried it on 256 and it goes into swap and crashes. It even crashed on an openvz system with 512 mb ram.
I have heard about i/o disk problems with xen although I have not experienced them.