I'm going to define 'rare' as any song with five or less full performances here:
http://www.u2-vertigo-tour.com/most-played-songs.html
(I was going to say less than ten, but the original poster said only once or twice, so 0-5 may be a bit generous ...)
1. Treasure (Whatever Happened To Pete The Chop?)
2. The Dream Is Over
3. When I Look At The World
4. Acrobat
5. Lost On A Silent Planet
6. Sixty Seconds In Kingdom Come
7. Bass Trap
8. Falling At Your Feet
9. North And South Of The River
10. Drowning Man
11. Luminous Times (Hold On To Love)
12. Wild Irish Rose
13. Alex Descends Into Hell For A Bottle Of Milk/Korova 1
14. The Wanderer
15. Zooropa
16. Lady With The Spinning Head
17. Where Did It All Go Wrong?
18. Cartoon World
19. The Silver Lining
20. She's A Mystery To Me*
21. Promenade
22. Your Blue Room
23. Slug
24. The Wanderer
*The stats for this one are a bit wrong. A large number of ZooTV performances are counted as full songs when they should only be snippets. I'm progressively working to fix this and once it's fully corrected, I expect there will only be about five instances of the song live in full.
Should this number be higher, my pick for the setlist spot is The Three Sunrises.