This was actually pretty interesting as each of the main series could be considered the best.
-Halo 1 set the standard and is beloved
-Halo 2 was many people's first online multi-player game
-Halo 3 ended the story in spectacular fashion (and was the first Halo I finished)
-Halo Reach had an amazing and tragic campaign
-Halo 4 takes the series to awesome new heights
And all of them have arguably the best multi-player of their time. Good thread, tbone!