It really depends on what you are looking for. Quality of the recording or quality of the performance or setlist. Personally, the best quality is a soundboard to me. But you may run into some conflicts with sound ratings on U2Start on that one because most people there seem to think audience recordings are better than soundboards which is mind boggling to me.
My suggestions off hand would be:
-May 6, 1983 Boston Soundboard (complete recording) Obviously the complete Red Rocks show is also a must have if you dont have it already.
-November 21, 1984 Dortmund Broadcast quality (incomplete) but its the best recording from the Unf. Fire tour period.
-March 21, 1985 Chicago Audience recording but pretty good quality and a solid performance. (The Bologna Italy show others have mentioned is also a high energy performance with good quality, I think Chicago is slightly better though on the quality of the recording).
-November 17, 1987 Denver Soundboard, best recording from the Joshua Tour period. Not sure which version U2 Start has but the best one I have heard was a remaster from a direct transfer from the pro shot called "Back In Denver".
-November 18, 1989 Sydney Not the greatest quality recording but simply an outstanding performance. Bomb threat before the show, alot of tense energy which translated to a fantastic performance. Desire, Watchtower, God Pt II and All I Want Is You from this show are some of the best versions they have ever done.
- Any of the Dublin 1989 shows. All are very good quality and decent performances. I do think the New Years Eve show is a bit overhyped though. Great quality as its a broadcast but after the fantastic Streets intro it falls flat to me, all the energy is used right at the beginning.
- June 11, 1992 Stockholm Audience recording, great quality, one of the few times I think the audience recording of this one is better than the soundboard that exists of it. The indoor leg had a different vibe to it so its good to have a recording from those legs also and I think this is the best of them. Honorable mention would be April 21, 1992 Tacoma, great show, good quality. One of Ed P.'s better shows.
-August 28, 1993 Dublin Broadcast quality, I think every U2 fan has this one, if you dont you should get it.
-November 16, 1993 Adelaide Soundboard from a pro shot, most people will probably say Sydney over this one but I like this performance better. A bit more raw, Edge's guitar work on Dirty Day stands out much better. Several versions are out there. There is one that is 2nd generation though from the pro shot, this is the one to find. Not sure what version U2Start has.
- Alot of a great recordings from Popmart, take your pick. My favorites are:
-November 14, 1997 Miami (soundboard taken from a pro shot) great quality, good performance, last time the song Miami was played.
- January 31, 1998 Sao Paolo, Broadcast quality, great performance.
- February 11, 1998 Santiago, Broadcast quality, probably my favorite recording from this tour for both quality and performance.
- May 15, 2001 Chicago (ALD recording, very good quality), I'm a tad biased on this one as I was there but the debut of 11 O'Clock and Out Of Control on the Elevation tour. Great perfomance, one of the best I saw on Elevation.
- November 25, 2001 New York, Soundboard from a pro shot, best quality recording from the Elevation tour IMO.
- May 12, 2005 Chicago Audience recording, good recording for an audience one, one of the better setlists of the Vertigo tour to me. I'm a tad biased on this one also as I was there.
-Euro leg has a lot of mediocre recordings unfortunately. There are a few exceptions. Gothenburg is probably the best of the aud. recordings from this leg.
- The Buenos Aires 2006 broadcast is great quality. Unfortunately Bono's vocals are pretty raspy for alot of the performance, especially at the beginning. Its unfortunate as this would be a near perfect recording if they had been.
-Saitema Japan, the last show there, the date escapes me. The audience recording is better than the IEM that surfaced of this show. Top notch audience recording of a pretty good show.
Wow, thats a long post! Sorry, but U2 audio is one of my major passions/hobbies.
I would also suggest if you have a high speed connection (which personally I do not) going to U2torrents. You will get the best quality of most recordings that are out there from that site.