Here are my opinions on the various options for a Summoner character:
Sorcerer: Best choice IMO. You get summon monster and planar binding spells. As a sorcerer, you have a limited number of spells known, but that isn't a real problem: first, there aren't that many summoning spells, and second, a summoner is very versatile anyway. Remember that you don't have to take every single summon monster spell, as the higher level ones can handily emulate the lower level ones. You can cast lots of spells, and use metamagic on them with no increased casting time (summon monsters are 1 round anyway). If only you could use persistant spell on summons... but we can use planar binding for that. Oh, as a sorcerer you have a high Charisma, which is very useful with planar binding. The class is simply perfect for a summoner. Just don't assume that your creatures will always do the work for you, and remember to take the usual sorcerer staples - magic missile, fireball, dispel and a couple others.
Wizard: A close second. You get summon monster and planar binding one level earlier which is very important, but the sorcerer wins in all other respects.
Druid: You get spontaneous summoning, but you get that with a sorcerer too. However, you can only use nature's ally, which are much less useful than summon monsters IMO as few of them have useful special abilities. You don't get planar binding or planar ally, which is a major weakness. The druid is better as a combat summoner; he has animal growth and nature's allies are quite tough already. He also gets more things to do outside summoning. As an overall summoning specialist, I'd still use a sorcerer though, simply because of planar binding.
Cleric: Much like the wizard, only worse. Like wizards, you don't get spontaneous summoning. You also don't get planar binding - instead, you get planar ally, which carries an XP cost. And, you can't summon creatures of the wrong alignment, while arcane casters don't have this issue. The big advantage is that, like the druid, you are pretty good outside summoning.
Psion: This is actually pretty good. To be more precise, I'd be a Wilder and take the astral construct power using the extend knowledge feat (or whatever, the feat which lets you get an extra power from any list). You get supreme versatility in defining your creature, and astral constructs are pretty tough in combat too, even though (like the druid) you miss on miscellaneous powers. Unfortunately, you lack planar binding or similar effects, and this gives the edge to the sorcerer again.