Halo 2 had the best campaign story (Arby and Truth were great characters until Halo 3 pissed all over their characterizations, especially Truth) and the worst campaign gameplay.
I think that this actually makes it the
best candidate for true remake, meaning redesigning encounters and such, instead of a texture and model swap style update like Anniversary.
A remake of Halo 2 could retain the story while fixing gameplay and pacing issues- no Sniper Alley, removing the boss battles ( or at least showing the Chief blowing up the Scarab and making Regret killable in more ways than "punch him in the face"), fixing the pacing issues (more time on Earth, an extra level in the Keyship), make Gravemind actually look
intimidating instead of a fleshflower and give better buildup to his entrance by spreading hints throughout Delta Halo and Regret instead of having him just appear Diabolus ex machina, and so on.