Peggy is absolutely a good pick. Okoye boosts the damage from each of her blue countdowns, so the two of them should pump out a lot of damage together.
On offense Medusa may be faster, but Okoye/Medusa is a lot more tempting to hit on defense. Since Peggy is boosted she will have more Health, thus taking longer to take out, especially if you also factor Strange's passive into the whole thing.
3* Thanos/Medusa is a good option on non-essentials. For essentials I would go with whoever has the cheapest damage ability. The damage itself doesn't matter as that comes from Okoye, so cheap aoe or super-cheap single target. You just have use your imagination with Okoye, she makes so many terrible characters viable. Just try to think through who could benefit from her the most.
Medusa is my personal fav though, just not the fastest option.
OJSP said: I’ve seen several people mentioning this and I just want to clarify something: Okoye doesn’t boost attack tiles. So, don’t put Okoye in a team with attack tile makes.