The druid can do everything the wizard can when it comes to summoning, plus he can also transform all of his minions into birds via Animal Shapes and fly to the target, then turn them all into raging elephants for free HP. Or he can just throw down 32 wolves instead. Wizard gets a simulacrum but it appears to me that the druid is overall better at the one-man army thing.
Edit: well, druids and necromancers are probably on par with each other.
Except use
gate. That is a very high level ability.
32 wolves killed by one
fireball or AoE attack. I like what the druid can do. Is 32 wolves better than 32 mephitis with temporary hit points if that route was followed? How many wolves could an army of mephitis kill? Access to
gate is a huge deal too.
Now cleric versus wizard I might hesitate a little bit. Clerics have easier access to outsiders with damage resistance and other powerful abilities. The cleric does have access to
gate and might with a amenable DM talk an outsider that serves her god into working for her without having to bind it. High level clerics are a little scary.
I feel like the summoning order is at high level is:
1. Cleric: Because fiends and celestials are powerful and no one can get more of them than the cleric.
2. Wizard:
gate spell allows them to bind something nasty.
3. Druid:
Earlier say in the 7 to 12 range:
1. Druid: More types of conjuration spells.
2. Wizard: See above.
3. Cleric: Not much to summon. Almost non-existent.
It's kind of strange how the cleric goes from not having summons to powerful summoner as levels progress. It seems some classes work that way. Some are powerful very early on like the fighter or barbarian, Some are powerful much later on like the wizard, cleric, and monk.
Some stay pretty solid throughout their run like the bard, rogue, and paladin.