I completed my Y dex back when pokebank first came out because once you had a rare Pokémon, you could have them all. It was great. I also completed my Alolan dex a while back. It was really fun reading all the dex entries and seeing the Pokémon slowly fill in.
My sister and I have finished the ORAS Pokédex and the Sun and Moon Pokédex! I have since dedicated my time and will do so to complete the Pokédex in every Pokémon game I play. Completing the Pokédex is very fun and is one way to make me feel good about myself!
I've started working on the XY Pokédex, and so far I'm halfway there! It is gonna be a bit harder because I only have Y, and not any other games, but I believe in the GTS and WT!
I've only completed the S/M Alola Dex. Other than that, there are too many Pokémon and I don't feel like catching 'em all. How am I supposed to catch 800+ Pokémon?
If you don't count event only Pokémon, then I've completed the OmegaRuby Pokédex. If you do, then I still need a Valcanion to finish it.
I have completed the Pokédex in every main series game gen 5 and on. The only other game I completed it in was Pokémon blue, took me 2 and a half years but it happened.