My character is a lvl 45 ranger/assassin type. Most of my "thieving" skills are either at or near 100. So I'm thinking of changing from the Thief standing stone to another one.

Right now my leading contender is The Steed Stone. Getting all that extra carrying weight helps my hording side. I like to collect everything I can get my hands on in a dungeon.

Any other suggestions?

The Lovers stone in my opinion since all skills increase faster but not as much as the other standing stones

Level 45 and still using one of the three basic stones?
I switch from the Mage/Warrior/Thief as quickly as possible. Otherwise you level waaaaay too fast.
But anyways.... The Steed is great, or The Lord Stone.

But basically it just depends on what you're doing in the game, and how you're roleplaying.
Certain stones are good if you're fighting a lot of magic users (Atronach) and some are good for fun (Ritual or Shadow)

Lord is my favorite. Universally useful no matter what you're playing. Even the most dodgiest of characters can get some mileage out of bonus armor and magic protection from time to time.

I'd go with Lord or Atronach. The 50% magic absorb is very useful, and the reduced magic regeneration isn't an issue if you don't cast any spells.

For a Thief, perhaps Serpent Stone?

My thief / assassin uses Shadow Stone. Very nice insurance against being detected, and is 100% quiet and instant effect, don't even need to swap weapons. Also, it really fits the theme of the character.

Those mentioning other combat-based stones are forgetting you are (probably) attacking from stealth. You rarely get attacked, so those bonuses will seem almost wasted.

As for Steed Stone, I dont really need because I'll go for the armor perk to reduce weight, and my theif even takes the Extra Pockets perk in the Pickpocket tree.

I personally went for the steed stone. The weight bonus is nice

The Ritual stone is the only one I'd use because the last thing I want is to level faster. And it's awesome because it has got me through some difficult battles that I couldn't have done any other way. And it's incredibly fun to raise a small army of bandits or draugr and see them go charging off to smote my enemies.

Lord is good for anyone. Atronach is also very good.

Steed or Serpent stone is good since your an assassin type.

My assassin uses the steed stone. the 100 extra carry weight on top of the extra pockets perk in the pickpocket tree is awesome. 500 carry weight with 100 stamina is a great thing. I've tried the shadow stone but I dislike having a once per day restriction, I prefer a constant effect.

The Lover's stone is always my Standing stone of choice.

Lovers Stone

Lord Stone is nice when you don't need to level anything up anymore