I have a self-balancing method to my GMing. If the PC is going to be rewarded with a really nice item, it's usually because he took it from the corpse of his enemy.
If he wants to get that +10 Sword of Quickness, he has to beat the baddie who is wielding it. In that sense, the opposing forces are at least somewhat equal to each other, but I like to go under the assumption that the PC party will win and get the goodies, so I try to make it at least a little challenging and interesting.
I'm not sure what the most powerful item I've given out (in terms of Potent, Most Potent, Artefact, etc. as ranked in C&T I, II, II and all the Companions). I give out items in keeping with the power level of the PCs and the baddies to keep some balance. It also depends on the PC using the items. +10 Sword of Quickness, +15 Boots of Traceless Passing, and the +25 Cloak of Camouflage are more beneficial to different PCs for different reasons. The PCs that have those items are between levels 4-6 so they are very beneficial for them.
I think one of my PCs received arguably the most powerful item ever rewarded in our groups over the 30+ years I've been gaming. +25 Shield Slayer Battle Axe - Treat all AT as AT 1. Magical Armour gets a RR vs. 50th level attack.
Add to that, about 3 years later (this was during the college time period of my gaming group), we had a massive 2-semester-twice-a-week epic campaign. The reward for my Cavalier was a potion that increased the Temp and Potential values of the Prime Requisite Stats by D6. He ended up with Str 108 and Con 102 which gave him choices from the Innate Stat Abilities Table.
Con: Tolerance - May take 150% of total hit points before becoming unconscious
Str: Shocking Blows - All melee hits yield double concussion damage (couple this with a 2H Battle Axe hitting on the AT 1 table and you have heavy damage.)