I personally think halo 1 was brilliant (still have it in my collection of xbox games), halo 2 didnt have a campaign as good as the first but it introduced multiplayer to the series, which is one of the best multiplayers in fps history (imo), halo 3 had a fun campaign especially with splitcreen and some friends, but halo 3 really excelled in its multiplayer, i think its the best multiplayer in the fps's this generation (tied with tf2), it has a huge variety of modes, lots of maps, the ability to create your own map layouts and then play them with your friends online and offline. Also halo 3 is the only good fps on the xbox360 with splitscreen online, if you have friends or siblings you play with, halo 3 gets a hell of a lot better, some of you may not like it because you and your friends never truly got in to playing halo 3 together.
I don't want to try sound like a halo fanboy but i think some people are giving this game too much hate without really giving it a proper chance or not having an open mind about it. And yes i know the halo online community is full of retards, but at least theres a mute option lol.