What stone do you feel gives you an advantage on Master difficulty?
The Lord Stone is my favorite (+50 points to armor rating, +25% magic resistance)

Atronach. 50% absorb, +50 magicka, and I use Restoration, so the two recovery perks make up for the penalty to regeneration, leaving regeneration equipment as a bonus.

If playing a mage character I'd go with Ritual Stone, Atronach, or Apprentice depending on whether the character has any aversion to necromancy.

For most other characters, the Lord Stone seems to be the most helpful, although for sneaky types the Shadow is a fun one.

Nice

Lady.

Using game-breaking standing stone buffs like Atronach and Lord seem to completely defeat the purpose of playing on Master.

Depends on the type of character I playing, but usually its atronach. Spell absoration is amazing, works wonders with spellbreaker and breton race. basicly immune to spells.
Otherwise the lord stone, ritual stone or steed stone even.

The lady stone for sure. Health and stamina regen +

Lady Stone for my vampire.

Lord is arguably the best stone in the game. Atronach can be nice but it can also absorp self-target spells and I prefer to just mitigate magic damage rather than absorb it. Lord + Agent of Mara + Breton = 65% Magic Resistance, and that's just passive enchants, no gear involved.

Everything else (for min/max, at least) is pretty useless.

Lord stone. It also makes sense for my character, as he's from a noble family.

atronach and lord stone. lord beeing the best.

if you go to smith and enchant yourself to godmode, the steed stone, or how it is called, is the best. no armorweight and more carryweight.

lady stone is for me one of the most useless ones. as far as i know regeneration is insanely low in combat

lord stone for me

Atronach Stone.