God, it's such a long time since I listened to any Hendrix! :o
First thing that springs to mind is "Who Knows" from Band of Gypsys, for some reason.... also love "Them Changes" from that album, although it's a Buddy Miles song... but let me think...
I always preferred Electric Ladyland to the earlier albums, and although it's not a very imaginative choice, I'm going to have to go for "All Along the Watchtower". It's a rare case where a cover version completely eclipses the original and becomes the definitive version. It's extraordinary. And I think it's one of those recordings where neither the song nor the performance could be any better - not just good, not just excellent, but 3 or 4 minutes of absolute perfection (other examples: "Ace of Spades" by Motorhead and "Don't Believe a Word" by Thin Lizzy)
I'm probably forgetting some great songs though....