I love Halo. But I pretty much just love the franchise's single player mode and story, although multiplayer's quite fun (although not my cup of tea).
Mostly because I'm a Cortana fanboy, because I love to follow the tiny little details and references that Bungie leaves along the Halo story, the complicated invidents that happen all through the tale, the stories of the books and the mysteries that end up becoming solved if you dig REEEEEEEALLY hard into the whole thing.
It's amazing, but Halo actually is a game with a very deep story and background. Most people just disregard what's around them, and fail to discover these details, instead just playing multiplayer matches over and over. Heck, I cried my eyes out when I heard "You found me... but so much of me is wrong..." in Halo 3, in the Gravemind level. And it took my breath away when the diary AI spoke to me through the final terminal and opened my way through, redeeming itself of all of its past wrongdoings.
...the problem is, most people don't really know Halo, so all they see is multiplayer being played by jocks and further degenerating the FPS genre into space marines over space marines. But the original concept was amazing and original, and all the games are still really good. It ends up having a large and complex canon, and the recharging energy, although annoying nowadays, helped the transition of the FPS genre into the consoles, something which still hadn't been done accurately until this game. Rapid medpack use and quicksaves through keyboard were no longer needed due to the regenerating health mechanic, and the game was still a challenge. Playing through Halo 3 in co-op legendary is still one of the best gaming experiences I've ever had.