It's not summoning, but an army of Poppets is quite affordable and an interesting take on the concept.
I think to be a good summoner you need to be able to cast summon spells as a standard action, and you need summons that keep up in level with the best spellcasters. That means a summoner, a cleric with the sacred sumnmons feat or the herald caller archetype, a monster tactician inquisitor or an animal shaman druid mainly. An arcanist with the occultist archetype can do so a few times a day (getting less often as they go up in level) and a wizard with the academae graduate feat can do so at the risk of being fatigued.
Of those a monster tactican inquisitor or an evangelist cleric is the best IMO, the inquisitor because they give their summons teamwork feats and the cleric because they can buff their summons two different ways.
Of note if you're wanting an army - the superior summoning feat gives you an extra creature on any spell which summons 2+ creatures. I haven't seen it mentioned above so also remember that the augment summoning feat is all but required for a character dependent on summoned creatures.
It's not required but the summon {good/neutral/evil} monster feats or some similar feats can expand the list of creatures you can summon. A few other feats like versatile summon monster, evolved summoned monster or summon guardian spirit can give your summons extra capabilities.
That's kind of what the Master Summoner archetype is for.
It's also pretty annoying to other players, so if you're going for this strategy, be prepared to be kicked out of the game.
Matthew's not wrong - the best counter is to be prepared with stats for the creatures (buffed and unbuffed if appropriate) already worked out before the game, and with basic strategies for what they're going to do worked out as well so that taking your turn doesn't take forever.
Yeah, there are hazards to this concept which are:
1) Your turn takes a lot of time, which annoys other players
2) In tends to be too effective, trivializing a lot of encounters, which annoys the GM
Basically any build which has a lot of minions whether they're Undead, constructs, summons, or anything else all suffer from these two issues which can make the game less fun for others.
I think to be a good summoner you need to be able to cast summon spells as a standard action, and you need summons that keep up in level with the best spellcasters. That means a summoner, a cleric with the sacred sumnmons feat or the herald caller archetype, a monster tactician inquisitor or an animal shaman druid mainly. An arcanist with the occultist archetype can do so a few times a day (getting less often as they go up in level) and a wizard with the academae graduate feat can do so at the risk of being fatigued.
Of those a monster tactican inquisitor or an evangelist cleric is the best IMO, the inquisitor because they give their summons teamwork feats and the cleric because they can buff their summons two different ways.
Don't sell the herald caller short. Their best ability can get ignored. "The herald caller and her summoned monsters can understand each other’s spoken words as if they shared a common language (though this doesn’t give summoned monsters the ability to speak if they normally lack it)." This allows you to have anything you summon do whatever you want. A favorite low level tactic is to summon a creature with grab and tell it to grapple your target. For instance, the Gar from Expanded Summon Monster has a +7 to grapple, before adding other modifiers. Add the Versatile Summon Monster feat and you can give that Gar the Aerial template to let it grapple creatures anywhere.