I have been mountain biking since the 80's and even did some racing until med school. Now I hardly have any time. I have a full suspension Cannondale hanging from the roof in the garage and mostly use my hardtail when I get to go out (it's lighter).
First you have to see what kind of riding you will be doing. For mostly flat trails go with a hardtail. If you don't mind the extra weight a full suspension will be more comfortable, especially for fast , rocky downhills. Most name brand bikes will have good choices. Make sure you get one with good components (Shimano LX has good performance/value) and go to a real bike shop, they will help you out.