Realistically, it depends on what style and level of involvement you are interested in. Are you just starting out? I found out that there is a marked difference between American Kickboxing and Muay Thai soon after I started.

When I was first getting into it, I took classes at Bodies In Motion, which, as an introduction, I would give a solid recommendation to. They're also all over the place, and there's probably one near you.

I believe that Benny Urquidez's place is out in Van Nuys somewhere...I've never trained with him, but he's got a pretty solid reputation, as far as I've heard.

Here's a link I found to him: www.bennythejet.com

I can't see the site myself (no Flash on my system here), but it should have some decent information.

Best of luck, and let us know how things proceed.