Famous examples of the LONG par 5 from championship golf include Baltusrol-17, Olympic-16, TCC (composite)-14, and Pebble Beach-14, and TOC-14. I haven't played Baltusrol or Olympic. Essex County Club-3 and Ekwanok-7 are also properly in this category and deserve to be mentioned among the best. (100 years ago, Ross built a 623 yards... If you somehow could adjust for distance inflation, what would be the equivalent in 2015 yards?)
When they were build, all of these were untouchables... no longer the case. I tend to think that if you were one of those who could get home, you'd be crazy to try and thus face a somewhat boring second shot.
Though I'm sure there are holes I fail to consider, the 18th hole at Yale (621 yards) stands out to me as the world's greatest long par 5. Like a few other holes at New Haven, it's completely unique in golf. I don't know of another hole similar built before or after. Unlike the other holes I mentioned, the second shot can be just as interesting, exciting, and exacting as the tee shot. I'm not sure that anyone but lefties, who hit right-to-left fades, could get it up fast enough to even get it on in two shots. Apparently this hole is controversial... Are there legitimate critiques or are the detractors simply unable to figure out what to do given that the other courses they've played, built by Jack Nicklaus, Tom Fazio, et al., caused an underdevelopment of their imaginations?