The optimum choice would be to make your selection, if not here is the order I would use:
Best of 1980-1990 // this would enable them to see some good old school U2
ATYCLB // familiar songs with good melodies
Bomb // Recent and therefore likely to enable them to enjoy "new U2"
The Joshua Tree // The classic
Achtung Baby // Introduce them to some modern gritty sounding U2
Unforgettable Fire // Now show them how U2 can be soft and melodic, not just rocky
Pop // Now they can see how U2 reinvent themselves and sound crazy
War // Show them the political themes and motivations that power U2
Zooropa // U2 at their experimental peak?
Boy // Show them how U2 were making classic songs at the age of 20
Rattle & Hum // U2 doing blues *sigh* but with the great songs of Desire, AIWIY etc
October // erm..beautiful..october
Best of 1990-2000 // show them how U2 suck at choosing good music
Or, perhaps a better idea would be to show them the DVDs. Its my personal opinion that U2 come across best live. When they see all those people, they will feel stupid for not liking them sooner.
I suggest:
Slane
ZOO TV
Popmart
Boston
Vertigo (they will be prepared for more talking now)
Rattle&Hum
RedRocks