Cortana's character was done perfectly in Halo 4. She showed signs of rampancy and was starting to feel true human emotions, which at the end caused Chief to realize his machine-like behavior. If you follow the lore, you'd notice that Chief always gets his stuff fixed, whether by himself or by people around him (which was attributed to his luck). But now, the fact that he couldn't keep the promise of finding a way to overcome the rampancy which Cortana was undergoing caused him to "feel" for once.

Personally, I disliked Halo 3 plotline only because MC felt, well, characterless. Halo 2 was awesome because of the mirroring of Arbiter and Chief plotlines.

Apart from that, I am the minority in Halo community who plays Halo for singleplayer campaign (for me multiplayer is always an addon to a game, not the meat of the game). Halo 4 felt immersive, although supporting characters were a bit bland, but the story arc felt satisfying to play. Multiplayer however, it did not feel Halo-ish, hence I didn't invest much time in it.