Actually, I'd argue in favour of the Cleric as the best summoner. It means you can take Celestial as a language and talk to them, which adds a lot to the versatility of the spell, and which the Druid can't do. And access to the alignment domains makes the duration sufficient for a low-level caster; you've got Bless and Prayer etc to buff your new allies, and being able to spontaneously cast healing means you can often just learn Summon Monster spells with spare spell slots.
Plus, of course, you don't have to take it as one of the precious few known spells that a wizard or sorcerer does.