Depends how you define a good year for an artist - but let's say it's about impacting Pop Music - then you can't go past 2008, which came off the back of a collaboration with producer Red One and resulted in Just Dance and Poker Face.
Depends how you define a good year really but lets say for Pop Music's sake it was 2008 - as it was off the back of collaborating with the producer Red One to launch the tracks Just dance and Poker Face