My favorite series thus far has been the LEGO series. Some of the games I haven't had much interest in like the Indiana Jones one, but overall, I've enjoyed the humor and the relative degree of difficulty in solving some of the puzzles and the execution of the games.
I also like the Burnout series. I was advised to try Burnout Revenge first and decided to get it after the demo. I have yet to really sit down and play it because I got hooked on Burnout Paradise. I haven't checked out Burnout Crash!, but I do plan to purchase it. I really wish that Criterion would get back to Burnout, but it looks like their Need For Speed games are really Burnout games under another name, so I'll have to be satisfied with that.
My latest favorite will be whatever Forza puts out. I demoed Forza 3, but never bought it. I went ahead and got Forza 4 and I'm hooked, so when Forza Horizon comes out this year, it will a first day purchase without hesitation.