Crothian said:
Mass Haste is a Haste spell for everyone. They all get +4 AC bonus, and an extra partial action that can be a move, a spell, or an attack.
Spells I like are Repulsion and Tensor's Transformation.
Mass haste is, in my opinion, the most universally-combat-useful spell in the entire PH, excepting
miracle and
wish which can spoof its effect. Yeah, there are spells with specific other uses that are extremely powerful (Gate, for example), but over all combats throughout a campaign,
mass haste is your winner. The more spell-casters you have in your party, the more powerful it is, but that extra partial action is useful for anyone. It's an extra use of a magic item, or more movement to get a fighter into position for a full attack... oh, and a +4 bonus to everyone's AC that stacks with almost everything. It's just too good.
And don't let anyone tell you that the "burn through your spells faster" effect is a significant mitigator, either. (And they'll try!

) When you're casting twice as many spells in a round, the combats are generally lasting half as long.
Since the bard in our party took
mass haste, we've had dozens upon dozens of battles, and almost every one of them has started off with said bard casting said spell. It's made the difference in a huge number of those battles.
This message paid for by the Wizards for Mass Haste Society
-Sagiro