Having some random lvl 61 person come up to me in RuneScape and give me rune helm and dagger, 50k, tons of wind and mind runes, tons of noted cooked swordfish, and about 55 noted gold ore. I've stopped playing for about a year now but that moment...I nearly fainted I was so happy. He logged out almost right after he gave it to me and I never saw him again (and I was still a noob, lvl 5, so I didn't get his screenname)
I've also made someone else's best videogame moment (or so he said to me at school) when he joined Runescape and I dumped my bank account onto him (Rune scim, 1 mil gold, a bunch of tunas, lobsters, and swordfish, rune pick, crapload of mind runes and wind runes, addy plate and legs, and pretty much everything else valuable to a noob). I haven't touched Runescape since, but right after the series of trades he said he almost wet his pants at the value of all the stuff.
Random high-level giveaways are the best.