Me personally, well at least my top three would have to be...

1. Steed stone - it's really good when you just want to get around and carry whatever you want and not have to drop everything.

2. Apprentice stone - I find this being extremely helpful (especially since my characters a Dunmer pure mage) and the drop back isn't that bad either.

3. Atronach stone - Again being a pure mage I love this.

Steed- Just for the weightless equiped armour.
Warrior- Just because the name of it.
Ritual- Thanks to Dawnguard this stone is really useful.

Only one. Magic. This is due to the % increase.

Warrior Stone in the beginning and the Lord Stone for the end game.

Atronach, Lord, Ritual.

Mage Stone.

I play Hybrid characters, Restoration and Destruction are two of the slowest Skills in the Game to raise.

1. Warrior - Guardian Stone : Warrior Stone / Standing Stone : Steed Stone

2. Mage - Guardian Stone : Mage Stone / Standing Stone : Ritual Stone

3. Thief / Assassin - Guardian Stone : Thief Stone / Standing Stone : Shadow Stone

Tower Stone, helps me bypass pesky locks I don't feel like unlocking for loot.

The Lover Stone or Warrior stone.

I have to admit, I do like that Steed stone. Nothing like weightless armor and a carry weight of almost 500.

Why are some people saying one or the other? Why not both?

I like using the Antronach and the Lord Stone at the same time.

Mage stone.

Lady to start, then Lord later on. I won't use the Guardians as levelling is too fast for me anyway, and I'd rather a safe resist than a gamble on a chance to absorb magic.