I'm going to answer the question: Are any summons especially good? Because I'm bored and have too much spare time, I'm going to rate all of them

Valefor: Decent as a starter, and has two overdrives. However, because of his low HP count later-game enemies take him out extremely quickly. Score: 6, 7 with second overdrive

Ifrit: Physical strength and defense along with HP go up very quickly, and decent speed backs it up. Useful, but nothing special. You'll probably use him throughout the game. Score: 7.5

Ixion: High attack and special attack power and extremely high speed, but defense suffers. Nearly undefeatable if it can learn Haste. You'll want to use him often. Score: 8.5, 9 with haste

Shiva: Well balanced, with very few weaknesses (despite her obvious weakness to fire). Decent attack and Magic attack, decent defense and Magic defense, and somewhat boosted speed. Overdrive gauge fills very quickly; take advantage of that. Teach her Haste and you have an invaluable aeon. Score: 9, 9.5 with haste

Bahamut: High attack and Magic attack, extremely high defense and magic defense. His speed is low, but he can stand the beating with his exceptionally high hit point count. Score: 9

Yojimbo: I hope you have huge amounts of gil with you, because otherwise you can't get him and you can't use him either. I hate him. You have to pay for a chance at an attack. There's a chance he WON'T attack. To make it even worse, he doesn't even have an overdrive. I never use him, he's not worth it. On the plus side, his summon animation is pretty. Score: 3

Anima: It's hard to control Anima, and that takes some getting used to. He's also a bit of a pain to get, but nothing horrible. However, once you get past those pratfalls, he's exceptionally powerful. Score: 9.5

Magus Sisters: Sure, they're a pain in the arse to get and sure, they're hard to control, but once you get past that they're unbeatable. Their overdrive is hard to pull off, however, so they're a bit undependable. Score: 9