The best ring in the game is the King's Signet Ring.

King's Signet Ring
Weight: 0.1
Value: 10,000
Effects;
Reflect 100%
Restore Health 10 pts.
Restore Fatigue 10 pts.
Absorb Magicka 100%
Constant effect.
How to get it:

Finish the Mournhold Storyline. If you helped Helseeth out, his final mission will be to find out something about Almelexia. Once you've beaten her in accordance with the Mournhold storyline, Helseth will have no more mission. Since he's obsolete, you can kill him.

---WARNING---
Guards in Mournhold do their duty to protect the king. If you fight him, they will ALL come after you, so prepare for a fight.

Kill Helseth, find his body under the mass of corpses, and loot him for the Ring, a Daedric Dai-Katana a Frostguard Robe, and some royal armor.