I'll agree that the Gilgamesh Mark Hunt is easily one of the best side quests in FFXII and probably one of the many awesome experiences in the entire series. His Blue Magic just kicks your ass...

I like how he's kinda been implemented in the ports for the earlier games as well. VI Advance had an awesome intro for him and he's a pretty fun summon. He's just hilariously awesome. Course he will always be remembered for the awesome stuff he does in FFV

Just for fun...
Gilgamesh Tribute 1
Gilgamesh Tribute 2