I'm a fairly big Tony Hawk mark and have all three...
I'd have to say American Wasteland is the best. The storyline is awesome (including the voices of Eric from Boy Meets World and the original Penny from Inspector Gadget), it's probably the best storyline they've done, imo. The controls are top notch and respond well. The levels are great and even finding all the gaps isn't that hard. You should be able to pick up a new copy on amazon for a reasonable price... with shipping mine came to under $11.
Next is Project 8, imo. Again, a good bit of fun, not counting a weaker storyline. The create-a-skater mode has more options (women and children), but the regular guy's is a bit weaker as a result. Proving Ground tries to be more serious and is great graphically wise, but I'm just not big on it overall.