Halo 2 is probably my favorite game. Ever. The Campaign of the game is enjoyable, and even if there's a flaw or two around the end, it's definitely excusable, since the game wears those flaws so well. It also leads into Halo 3, which is just as good and debatably better storywise. The Multiplayer is near perfect, with amazing maps as well as glitches that made combat even more fun than it already was!
Within the campaign is a very well crafted story, along with a very tightly knit weapon sandbox. Every gun serves a purpose. 2 also brings THE most frustrating difficulty in any FPS ever. Legendary is already hard enough, with enemies that melt you in seconds, snipers that could one shot the you without warning, and many other factors that make the game complete torture. Factoring in the modifier skulls, you get the unofficial difficulty "LASO" (Legendary All Skulls On), where the game becomes the ultimate endurance test! Many complain about how hard the game can get on the hardest setting, but it IS the hardest difficulty, no matter how "artificial" it seems. You got what you wanted! Plus, Gravemind is hilarious on LASO.
On the other hand, Halo 2's multiplayer on the Original Xbox is basically what birthed the modern online FPS.. Like really, if the Original Xbox was more popular, I could easily say that a lot more people would set Halo 2 as their favorite in the franchise. The multiplayer is fantastic, and is only made better by button combos such as the BXR and Double Shot. The game is just as blessed by imperfections from the development process as Super Smash Bros. Melee
, and only benefits more from the FPS setting. Button combos add to the game more than they detract, and it's sad that they turn some away from the amazing multiplayer. There were also glitches that allowed for Super Jumps, which make for a some hilarious games of oddball. Aside from that, the modes are the same as they appear in the other Halo games, and are just as satisfying! 2 Is also where the Franchise's map design peaks, with the additions of fan favorites Zanzibar and Headlong. In my personal opinion, District is the best map in the franchise. I could be considered a genius if the knowledge I had of the map went to something... useful. Overall, everything in the multiplayer is perfect.
Many games do have imperfections, and Halo 2 is no different, but I don't think any other game will have imperfections that work for a game as well as they do here.