Difficult to say, since many factors of one game will compliment the other when it comes to pros and cons.
Story-wise, S/M > USUM > X/Y > ORAS. Sun/Moon did a lot of new things with the series that helped it stand out from the many instalments of franchise fatigue such as the Aether twist (as predictable as it was) or the championship battle against Kukui. USUM returned to many standard elements that weakened its appeal, though still performed things better than X/Y's cookie-cutter story. ORAS is last due to Gen 3's story in general perhaps being the weakest of the franchise's main instalments due to Magma/Aqua's poor motivations.
As far as the post-game content goes, the Looker mission in X/Y is easily the best of the bunch, though the game leaves you with far less to do after that than the rest. Gen 7's post-game is a bit better with an introduction of a far larger area than what X/Y offered and ORAS doesn't have too much going on after the Delta Episode.
Character-wise, USUM > S/M >>> ORAS ~ X/Y. Gen 7 had some of the most developed characters in the franchise such as Lillie, Guzma and surprisingly USUM Hau, with Lusamine's motives in USUM also being a vast improvement from Sun/Moon's crazy bitch characterisation. ORAS doesn't really do anything to break the mould and its villains have perhaps the dumbest motivations of any team leaders (how did they expect to live in a world either entirely flooded or devoid of water?), though its more memorable characters still have more going for them than X/Y's generic cast.
As far as gameplay goes, things get more complicated. Gen 7 has better ease when it comes to certain things like EV training or Shiny hunting, or get rich quick schemes through the replayable nugget battle in Po Town as well as making move set up easier with how simple it is to get heart scales. Gen 6 was better in the introduction of Mega Evolutions which were a far more viable change to battle than the cheapness of Z-Moves, though all but 3 (2 if not doing the Hidden Ability Torchic event) are available for a good portion of the main story and it wasn't until ORAS where they were really utilised much for a main story playthrough. As for catching non-Regional Pokemon, Friend Safari was good but very limited for a game that offered the National Dex and the Mirage Islands worked far less as an organic part of the world than USUM's Ultra Wormholes. Either USUM or ORAS would take the win in that category, considering the rustling grass Pokemon were sometimes non-Hoenn ones that allowed for it to house more Pokemon than any previous game.
I'd probably say Gen 7 > Gen 6 overall, though Gen 6 has more replay value mainly due to Gen 7 having a few new Pokemon that couldn't be transferred between Sun/Moon and USUM such as Ash Greninja or Necrozma's fused forms.
