The unreleased Sun Studios tracks from the R&H era(She's A Mystery To Me, full band version of Can't Help Falling In Love[different from the version released in the 90s], Bottle Rocket).
The Zooropa sessions as mentioned before - Wanderer w/Bono vocals(one of the crown jewels of unreleased U2 imo), and the Zooropa versions of If God Will Send His Angels, If You Wear That Velvet Dress, and Wake Up Dead Man.
I read once that Bono said there was a version of The Playboy Mansion during the Pop sessions that had a completely different set of lyrics that weren't as dated as the ones that were used. I'd like to hear that if it exists. Also, I remember reading somewhere when the Best Of 1990-2000 came out that The Playboy Mansion had been re-recorded(with Numb, Discotheque, and Starting At The Sun), but not included on the release. I'd like to hear that as well, as it might include the aforementioned different lyrics.
The early version of Original Of The Species that was heard during a 60 Minutes interview with Bono well before HTDAAB was released.
The entirety of the 2003 Chris Thomas HTDAAB(including Mercy).
The earlier NLOTH tracks.
The SOA tracks.