Depending on the specific needs, I'll recommend one of three options, all of which I have worked with in the past.
1. Shared Hosting - you can't beat 3Essentials for performance, support and cost. They are great, and with Plesk you can setup multiple clients on a single reseller account with ease. (Referral link in my signature)
2. VPS Hosting - Contrary to what is mentioned earlier in this thread I've found that a VPS server is a very viable option for some hosting scenarios. All of my sites are actually hosted on a VPS account with HostMySite.com and I have NO issues at all!
3. Dedicated Server - If you want full control, and responsibility, then this is the route to go. It will be expensive, but in the end it typically pays off. I strongly recommend RackSpace for this, as their support is awesome.
Overall it really depends on what you are doing, what your expectations are, and what level of client support you need when hosting your client sites! Drop me an e-mail if you want to discuss directly msellers@iowacomputergurus.com