This is an interesting topic indeed. I'd agree with BW about success in the personal sense, but I take Calliope's question to be what do newspaper writers mean when they say "flop". I thought, especially with regards to Broadway, "flop" means a failure at the box office. I've often read "the show was a flop even though it was excellent, ahead of its time, etc." or "The critics loved it but it bombed at the box office."
Nancy Reynolds deals with this question a bit in "Repertory in Review." One example I remember is "Divertimento No. 15" which she said "the dancers fought for for 10 years." It wasn't popular. Now (perhaps because it's a small-scale tutu ballet) it's in the repertories of several small companies.
Two of Balanchine's ballets that were box office "hits" were "Jewels" and "Vienna Waltzes." Neither are probably his finest work, and "Vienna Waltzes" is, at present, not very popular -- in five years, it, too, may make it bigtime on the "full-length ballet" circuit.
Kenneth MacMillan's "Manon" is another curiosity as flops/hits go. A flop when it was new -- dismissed by critics, not very popular with audiences -- it's now firmly entrenched in the Royal Ballet's repertory and viewed as a "classic" -- and quite popular. This fascinates me, because I can think of a lot of ballets that critics loved and audiences didn't like, or vice versa, but I can't think of another one where almost everyone was BORED when the ballet was new and now many people love it.
Then there's Nutcracker, the ultimate ballet success -- even though each version is different.
I think Morris Neighbor's standard of when a ballet makes it into other repertories it's a success is a good one, although one has to be careful here, too, because then we get into the mass market question. Does a classical ballet have to be a mass market success? I don't think so. I think some of the ballets that really only work on their home turf were/are quite successful -- Ashton's "Symphonic Variations" is one, Bournonville's "Folk Tale" is another.
Finally, when does the statute of limitations run out? "California Poppy" was hugely successful in its time; it may not be revivable now, although another Pavlova may come along any moment now

Estelle, what is the Gardel ballet that has the most performances in the Paris Opera repertory? Long gone, and long forgotten, but a success. Bournonville's most successful ballet was Valdemar, flushed in 1929, but beloved until that time.