Major Image, of course. Nothing quite as flexible as a good illusion, especially when you can add the thermal component to your illusionary Walls of Fire...
It's true. Higher levels may have more powerful spells, but 3rd level is the coolest!
I chose Haste because the last time I had an NPC use it against the party it was one of the most fun fights we've had this campaign (it was actually a cleric with a potion, actually). 2nd level's coolest spell is Web, it's nasty.
You could take away all the other spells and it would still be D&D. But Fireball? No. That spell is quintessential to D&D.
It isn't about which spell is most useful. It is about someting far more profound than that. It is quite simply unfathomable to think of D&D without the good ol' Fireball.
Session before last our wizard cast it *twice* trying to dispel a spell that had half our party out of action including our best fighter. Guess what? He unsummoned the celestial dire bear my cleric had just summoned and dispelled haste on her. Then went on to dispel half the beneficial spells I'd cast on the party. Didn't even touch that firk-ding-blasted gluttony spell on any of them.