At high level, Druids have some fantastic "kill an army" spells. At mid-to-high level, Druids are the toughest melee opponents, thanks to Wild Shape and self-buffs. At low-to-mid levels, Druids have the best battlefield control spells -- from entangle[/u] to wall of fire, nothing can compete until Wizards get wall of force.
Druids can summon elementals earlier than Wizards or Clerics, and those are often thought to be the best things for even a Wizard or Cleric to summon. And Druids do it spontaneously.
In terms of raw power, Druids are the top.
They lack several things:
1/ Strategic and tactical teleportation;
2/ Plane shift capability;
3/ Funky save-or-suffer spells ("debuffs"); and
4/ Cool outsiders for long service (like planar ally or planar binding)... of course, Druids can make these kinds of friends via awaken, so whatever.
Cheers, -- N