1) Although many soi-disant "best of" packages are really "greatest hits" packages, obviously a group's hits are not necessarily its best recordings--and this is very much the case with Jethro Tull.
2) On the other hand, in regard to Jethro Tull the point is moot, for Jethro Tull had only three (U.S.) hits, "Living in the Past", "Bungle in the Jungle", and "Too Old to Rock and Roll, Too Young to Die", hardly enough to fill an album.
(By the bye, in the period in which Jethro Tull sold records in significant quantities--the vinyl disc period, that is--, a hit record was a 45 rpm disc, distinct and separate from the 33 1/3 rpm LP its song may also have appeared on. In other words, to have a hit then, you REALLY had to have a hit, you really had to sell a great many actual copies of an actual record.)
4) Now it just so happens that, though the group has been continuously active from 1968 to 2003, the best of Jethro Tull was Jethro Tull from 1969 to 1973, as captured on the albums "Stand Up", "Benefit", Aqualung", Thick as a Brick", and "A Passion Play". All of these albums are of even and consistent quality with no particular stand-out tracks.
In view of points 1), 2), 3), and 4) above, it seems to me this record ought to have concentrated on the exceptional occasional good tracks from "the period of retreat", from post-"Passion Play" releases (with "My Sunday Feeling" from "This Was" thrown in for good measure). It could have included, for example, "Skating Away" from the otherwise dismal "War Child", "Moths" from "Heavy Horses", and, oh, say, "Pibroch" from "Songs from the Wood".