Completing the X and Y dex was lots of fun. My girlfriend has Y, I have X and one weekend we decided, to complete our dex. Both of us. So we took a sheet of paper and wrote down all the Numbers of the Pokemon we were missing. Which were about 300 each. And then we went hunting. We colour coded which ones each of us got, we stalked the GTS, we battled like crazy to evolve Pokemon, we went hunting for version exclusives and other wild pokemon, we were missing.
We each had a box of pokemon for trading and once that was full, we traded 30 pokemon back and forth. It took the whole weekend but it was super satisfying to scratch more and more numbers off the list and in the end, we both sat there with a completed dex. It was an amazing experience and lots of fun!
I've never ever in all my time playing through the Pokemon games completed a pokedex. Ive definitely come close but I've never been able to say that I've fully completed one. Kinda sad actually if you think about it!
I'm thinking about getting my hands on a copy of ultra sun or moon (not so sure which one yet) and I'll definitely be trying to complete it on there!
I haven't yet, but my friend is gonna help me since I have Omega Ruby and he has Y. We're gonna start trading tonight, and hopefully I'll have it done before I leave on Sunday (I'm only missing 144 including mythicals, so it shouldn't take too long).
I used PokeGen to complete it in Black 2 for that shiny Haxorus, and then I transferred them all over to X to get the Shiny charm since I was an avid shiny hunter then, but that's it.
In AS I finished the whole thing normally, in Moon I just did the National Dex, and then in UM I made a livedex to give me something to do since the plot was pretty much the same as SM, anyway. Of course, now I could transfer them all to Moon, anyway, but I'm too lazy (and scared I'll forget to renew my Pok?Bank subscription).
Almost the Kanto region, gen 1. I had all Pokemon except for Mew and that was frustrating. There were a lot of rumors floating around about how to get Mew, but none of them ever worked and there was no YouTube back when I played it to show you how to obtain Mew. I never really tried in any other generations after that, it was way too frustrating and now there are so many more Pokemon! Ah!
I have only completed the pokedex in Pok?mon X and that was because I wanted the item that helped breed shinies. I was so proud when I finally hatched a shiny Budew. It is probably the only game I have legit shinies in.