The only real option here is Grant.
A vote for Greeley or O'Conor is a vote to turn the South back over to the white supremacist Democratic Party and destroy everything that the war was fought for. The only person capable of fighting the Klan and suppressing the enemies of the Republic is Grant, and that's who I would vote for.
I vote Greeley, mostly out of sympathy for him. I think also you're confusing Greeley with Horatio Seymour. Seymour was a racist and white supremacist who not only hated blacks, but natives as well. Greeley was nothing of the sort. He was a Republican, the same as Grant and he was an early advocate of something I think you also keenly advocate, socialism. He broke with Grant mostly due to the scandals. On politics, they were fairly close. The Democrats supporting Greeley was a cruel irony, considering that he had spent years denouncing them.