It is hard to say now as it is early in 4's life, but I can say I had more fun on 3 just because everyone I knew played it and customs and matchmaking were a blast.
This one is pretty fun even though a fraction of my friends play it, so it is hard to say right now.
Also, I should make it a note that people who value the older games so highly is due largely to nostalgia. In our lives we tend to remember happy memories and repress bad ones, basic psychology, yes? People essentially have good memories of the old Halos while their bad memories of the games have faded quickly. The reason why people value the current game lower than the other ones is because the bad memories of the current game are fresher in their minds, so obviously the nothin-but-good-vibes Halo 2 takes precedence over the now it's-alright-but-there-are-problems Halo 4