This topic reminds me of what people said about Zelda:Ocarina of Time, many people say that Ocarina of Time was so overrated and doesn't compare to modern RPG's and newer games. But the thing that Zelda and Halo 2 have in common, is that they both revolutionised different properties of what gaming is today. Zelda revolutionised in-game exploring and Halo 2 revolutionised online gaming. They may not be as great as modern games today but they both changed the way games are being made.
In my opinion Halo 2 was the best Halo game in the series and yes it had flaws, every game has flaws, no game is ever perfect. But it was a very special game that had a solid and unique campaign, breathtaking multiplayer and many extras that gave the game lots of replay value. These extras include the scarab gun, secret soccerball, out of the map glitches, ghost launching and many more. It also had the best maps in the series including Ascension, Beaver Creek, Turf they all had a lot of balance and were a blast to play.
I really liked Reach's campaign, forge and firefight but I don't think that it's multiplayer even compares to how great Halo 2 and 3's multiplayer was.