Can someone recommend a great web host provider?

I’m currently with 1and1. I have a quad core, 4GB RAM, private server. My WordPress powered website receives about a half-million visits a month.

I’ve had a great experience with 1and1 for the 5 years I’ve been with them. I started off on one of those $5 pans and moved up to a private server over time as my website grew.

Lately I’ve had speed and server stability problems.

I’m not dead-set on switching providers right now, but I was wandering what other high-quality web host providers are out there.