A tossup between the various things that break the laws of thermodynamics most (that is, the stuff that can cause perpetual motion or endlessly create matter)...with the unlimited power I could get from making a motor out of a coil and a set of ring-gates, I could do a lot.
Other leading candidates would be a Tome of Clear Though +5, Tome of Understanding +5, or Celestial Armor (the super-light chain that lets you fly).
Below those three would be...a ring of regeneration. That would just nice if not anything more significant.
Wait! Wait! Stop the presses! I just remembered what the ultimate magic item in real life would be!
An Immovable Rod!
Yeah, that's what I'd want, more than anything.