Halo 3
One of the biggest musical scores of all time. Legendary and the only game that I played in the legendary difficulty. For me Halo is Microsoft's version of Star Wars. It combines great characters, actors, setting, soundtrack and gameplay. The design of the Ark is beautiful and much more enticing the Death Star that is just a sphere.
The gameplay clearly evolved from Halo 2. The pink needles gun can no longer be dual wielded. It was overpowered to do that in Halo 2. New items, better level design. Great levels to travel by car and airships. The soundtrack has changed and it seems to better match each level. In previous games it sounded more like a movie and wasn't strongly related to what was happening in the game. The last part with the Arbiter and MC running from the collapsing Halo's structure is just epic. It pays tribute to the last part of Halo CE with a much better level design.
The monitor's voice acting is one of the best acting of all time.
I didn't like Halo Reach's level design because it felt confusing, wide open and the buildings were all empty inside. Halo 3's level design does a better job at guiding the player. The levels designed for vehicles are clearly better in comparison to Halo 2. I also didn't like how in Halo 2 we get to play as the Thel Vadame and in when we finally get to see how the story is going to end with Halo 3, all we get of Thel Vamade is an NPC that we almost have no interaction with. There also very few lines written for him. I really wanted to see him talking more or interacting more in Halo 3.
I disliked some of the design decisions made, hence I don't see this game as a master piece. UE3 and Gears of War pushed the hardware more than Halo's proprietary engine. The AI makes some mistakes in some parts of the game, getting stuck for no reason. In the Cartographer mission, at the end, I was killed by the incoming ship. In the mission to blow up the reactor the game didn't tell me that I had to use the grenade launcher. The arrow that indicates where to go is too small. It doesn't feel right to rifle-butt to make the brute's armor drop off. In the mission to shut down the shields you go inside a tower and there is a checkpoint after a lift that forced me to rollback. I had the wrong weapons there and I couldn't retreat to swap the weapons. The only way was to restart the whole chapter.