What's your budget? What is your intended use, and what bodies/lenses do you own and plan to use on it? How tall are you, and do you want an eye-height tripod or something shorter/taller? These are all critical factors in determining which one to select.
There's a saying about tripods. There is light, stable, and cheap- and you can pick any two of the three.
I use a Gitzo Series 2 mountaineer (2531) coupled to a Markins M10 ballhead. I have attached the RRS Lever clamp to the ballhead and use a matching L-bracket for my camera body. It's not cheap (the two attributes I chose were light and sturdy
) however it's got the performance to back up the price. It is one of my favorite pieces of photo gear, both in what it has given me in terms of images, and the enjoyment I get from using it.
If weight is a big concern for you, going with a carbon fiber tripod is the best way. If it is not a huge concern then I would recommend buying a cheaper/heavier set of legs and getting a good ballhead.
There's a good article on buying tripods that you should read to give you some thinking points. The specific gear recommendations in it are a little dated as the models suggested have been replaced by updated versions, but the philosophy is still applicable and is the important part of the article. Think about where you want your tripod to end up, and if you can't afford to spring for that ultimate set right away, make some smart purchasing decisions now so that you minimize the money lost in incremental upgrades.
http://www.bythom.com/support.htm
Hope that gets you started.
Ruahrc