Gosh, I know I got the idea for today's screed from an online friend, who had offered thoughts on druids who summon animals. But I can't remember who, and I seem to have deleted the e-mail. Sorry!
Anyway, it is a great topic -- how do you make the best conjurer? Well, here are some ideas.
The Augment Summoning feat is essential. Anything that boosts your summoned minions is valuable, so adding +4 Str and +4 Con (without wasting another action) is a sure winner! The required Spell Focus (conjuration) feat is mostly useless for divine conjurers, but wizard and sorcerer conjurers benefit a variety of attack conjurations, from Grease to Incendiary Cloud. In fact, arcanists may well benefit from Greater Spell Focus also.
But attack spells are not your forte. Rather, your primary goal, after summoning your minions, is to make sure they last as long as possible and do as much damage as possible. That's why the best conjurer casts buff spells to boost the capabilities of allies and minions -- especially buffs that affect many rather than just one. Bless, Prayer, Magic Circle, Mass Buffs like Mass Cat's Grace, Rage, Haste, Good Hope, and others are all excellent choices because they affect several allies rather than just one per casting.
If you do have the luxury of helping your minions one at a time, then use those Cures, Aid, Protection From Evil, Shield of Faith, Mage Armor, Stoneskin, and more!
A good way for a cleric to boost the time that summoned minions stick around is to take aligned domains. If you have the Good domain, you cast Good spells at +1 caster level. Same for Law, Chaos, and Evil. Summoning an aligned creatures makes the summoning an aligned spell. So a cleric with Law and Good domains gains +2 caster levels summoning lawful good minions. That's two extra rounds of fighting, and when you are first level, that means three rounds instead of just one!
And if you are casting spells that affect all allies, you make better use of your spells if you have many allies. Similarly, if you have the Augment Summoning feat -- and you better! -- then that boosts all the creatures you summon. So it is to your advantage to summon many creatures rather than few.
With Summon III and higher -- Summon Monster III+ and Summon Nature's Ally III+ -- you can call your choice of just one top-potency creature, 1d3 second-potency creatures, or 1d4+1 third-potency creatures. In many cases, the 1d4+1 option is better. That's especially true if you are using a Metamagic Rod of Empower, as I detailed in a previous screed.
And the nifty thing about summoning many allies, buffing them, healing them, and then summoning more of them, is that you can act without putting yourself in harm's way. You are not on the front line. You are also not casting any "Look at me! I'm a wizard!" spells, like fireball or lightning bolt. In fact, you are not harming anybody, so you can act under the influence of spells like Sanctuary and Invisibility without restriction. That Invisibility is so useful, a cleric conjurer might choose to take Trickery as a domain just to gain access to it. Or, when you gain enough wealth, that Ring of Invisibility is a treasure indeed!
You really want to summon the most potent creatures possible. That means you want to cast the highest possible level spells at all times. Each higher-level Summon spell is much better than the one under it. Summon Monster II is better than Summon Monster I, and Summon Nature's Ally IX is better than Summon Nature's Ally VIII.
That means you want to stick to one class, without multiclassing. Even with the Mystic Theurge prestige class, it is not worth it to multiclass. A Cleric 9 can cast Summon Monster V; a Cleric 3 / Wizard 3 / Mystic Theurge 3 can only cast Summon Monster III.
So if you are going to stick to just one class, which should it be? Sorcerers gain higher level spells a level later than wizards, without any gain in return -- the conjurer (wizard specialist) has almost as many spells. So conjurer beats sorcerer. But even the conjurer falls down on the buffs and cures front -- he can't effectively keep his minions alive.
The druid is an interesting choice, because druids can Summon Nature's Ally spontaneously. That lets them take buffs and cures, switching out for summons as needed. At fifth level, a druid can use the Natural Spell feat to stay in an innocuous small (or, later, tiny) bird form while casting summons and cures. The druid has some decent single-creature buffs, like Barkskin and Cat's Grace; at higher levels, the mother of all mass buffs is surely Animal Growth, and at sixth level, Mass Bull's Strength and its siblings are quite useful.
Also, think of the druid's animal companion as a super-potent summonable minion with a permanent duration and -- if it dies -- with a casting time of a day to resummon. If any conjurer has reason to buff up a single minion, it is the druid and her companion. Especially since, by sharing spells, the druid benefits from all those buffs too. Sure, the arcanists have familiars, but they are piss-poor in combat and drain the master of experience points when they die. Not to mention that you can't call another one for a year. The druid loses nothing if the companion dies and can get a new one in a day.
Still, I think it is the cleric that really shines as a conjurer. The good/evil and law/chaos domains give a lovely caster level boost. The orange prism Ioun Stone that boosts caster level by +1 is worth 30,000 gp, so a +2 boost is pretty darn sweet! And Sanctuary lets you summon, buff, and cure without fear. The cleric's boosts and buffs for allies are the best in the book, from Bless to Prayer to Holy Aura. And being able to cure your minions really pays off. Take Celestial as a bonus language, and you'll be able to communicate with your minions for free (the druid would need to use a Speak with Animals spell).
And keep in mind that clerics also gain access to helpful minions via Lesser Planar Ally and Planar Ally. Throw in the Thaumaturgist prestige class, and it is very hard to trump the cleric as Conjurer Par Excellence.
But do not let power-gaming overrule your role-play whims. My liontaur druid/bard/seer (a homebrew mystic theurge variant) is not optimized for power! But he is looking forward to summoning nature's allies and then using nifty bard stuff like Rage, Good Hope, and Inspire Courage to buff them up.
Maybe the next time you're in a fight, you'll consider summoning some friends to do the messy stuff for you!