Wouldn't call that weak myself, I'd call that a cliffhanger ending. If any game had a weak ending I'd say it was 3; plot threads were resolved but the deaths for Johnson and Miranda were far from satisfying. Felt like cheap kills for easy nostalgia points in people. Also, having Cortana screech in your ear and halt your gameplay got old FAST. The developers killed replay value in the campaign not only because levels were just a lot of backtracking, but also because Cortana killed all momentum of a fun level repeatedly.
But, we can agree to disagree.
I thought the Tartarus boss fight was okay. I understand criticism of it and won't defend it. I also thought Chief's last level was appropriately awesome; you got to see how High Charity ended up overrun and all the chaos that was ensuing. If Chief was on Regret's ship for the final level and he killed him, then 3 doesn't happen. He was essentially the final boss of the series after Tartarus died and the Elites were on your side. I'm way too tired to write an alternative ending for the game I'm afraid!