Rock Band 2, but I have not played Guitar Hero World Tour.
GH1 was groundbreaking but incredibly clunky to play now. (The hammer-ons/pull-offs are a lot harder).
GH2 was all-around excellent.
GH80’s was what it was, a good add-on game not worth the price of a full one. Some of the songs on it are awesome.
GH3 probably had the best song list of the original series, but I hated the (mandatory!) battles and didn’t like the way the timing changed. And I never could beat the Slayer song. And the game felt a little rough around the edges in general compared to its predecessors.