Having just played and beaten Halo 1 and 2 ( preparing for 3 ) I'd have to say that the Half Life series is much superior. While I dont like Half Life 2 as much as others I do still feel that the original Half Life is one of the best FPS available on any console ( only second to Deus Ex ). Halo definately set the standards on how a modern FPS should be made, but I felt the level design was overly repetative in Halo and in various spots in its sequel. The story isnt as endearing as it was made out to be. Although the game definately kept the focus on action rather than taking breaks for stupid platform jumping, which plagued Half Life I admit and should be avoided in all FPS.