Now here's a two-album comparison I can really sink my teeth into! I've been listening to these albums for 25+ years and know them both backward and forward. Both can be ranked among the "Top 3" albums each band released, but neither is "the best" (IMO: Thin Lizzy's best is Bad Reputation, Aerosmith's is Get Your Wings).
Jailbreak is best known for the title track and the monster hit "The Boys are Back in Town", and rightfully so. Most American AOR listeners know the band by these two songs only. But if you listen to the rest of the album, there's a lot a quality, high-class material. "Angel from the Coast", "Warriors", "Fight or Fall", and "Cowboy Song" all evoke different atmospheres, but can all be considered classics. "Emerald" is the heaviest, most vicious thing the band has ever done! Only "Running Back" and "Romeo and the Lonely Girl" can be considered sub-standard, but neither one ruins the album. Back in the 1970s when bands had to release a new album every year, songs like these helped fill out an acceptable running time.
Rocks, on the other hand, did not have a huge hit, although "Back in the Saddle" and "Last Child" did receive some radio exposure. Still, EVERY song is an accidental, yet flawless miracle. While the album's production features some rough edges and intentional sloppiness (no doubt influenced by the amount of simultaneous partying), Rocks has become one of those definitive late-1970s musical statements. If someone were to ask, "What was it like back then?", one could do a lot worse than listening to this album. "Rats in the Cellar", "Combination", "Sick as a Dog", and "Nobody's Fault" should all be a lot better known than they are, and emphatically capture the zeitgeist of the times.
So, between the two, I would have to give a slight advantage to Rocks for being more definitive and more perfect than Jailbreak. Interestingly, both of these albums were followed by major blunders (Draw the Line vs Johnny the Fox, anyone?) that almost (but not quite) derailed their respective bands' careers. In closing, I would like to add that Jailbreak has the better album cover between the two, and that I will always wish that the 1979 movie "The Warriors" had a Thin Lizzy soundtrack!