The problem with this question is the answer "it depends." I've done a lot of summoning work, my character Valgrim the Malconvoker and my Summonable Monsters List is cited a lot on 3.5e CharOp threads.
There's a lot of options, it's the same as asking what spells you can use to buff any party member or ally. But what you need to do boils down into a couple areas regardless, and runs afoul of one key rule.
You can't do what you're talking about past level 5
Summonable monsters don't keep up with raw combat power after about SMIII, regardless of how much you buff them. At level 13,for example, you're using SMVI to summon CR8 creatures, while you're facing CR 15 creatures. They're just a speed bump, not a credible melee threat. A +4 to CON or a +1 to-hit makes exactly zero statistical difference once the differential grows that large. It just doesn't pay off. You have to go for things with special abilities, in which case the buffs you'd use depend on the exact creature and special (to e.g. increase DCs), which in turn should depend on what you're facing (targeting low saves, etc.). If you're fighting giants you try to get something that targets Reflex saves and then you buff whatever stat improves that special for that summon.
Smarter opponents ignore summons. They know what they are and they'd rather maneuver around them to get at you instead of meleeing with them. So any buff that makes them bigger, to take up more raw squares, or forces creatures to fight them (few and far between), help. Summoning an Ankylosaurus with SMV means you have a Huge wall of hit points between you and the enemy. Of course unless you're careful with positioning it's easy for one to soak the AoO and the rest to just run/Acrobatics past. Using grapplers like the kyton and octopus and buffing the grapple can help. Dumb opponents, maybe you can swarm with more lower level creatures, but even they tend to stop chomping on non-threats if someone else pours damage into them.
So in conclusion - there are no optimal buffs for combat summons, because that's not an optimal path. At low levels use whatever you'd use on everyone else in your party, mage armor, magic fang, bull's strength, etc., capping out with haste. At higher levels, you're looking at blockers at best front-line wise, you're better off summoning a shadow demon to possess one of those enemy giants than trying to summon critters to fight them.