Afaik in gen 1 I was only one mon short, or maybe I got the diploma and I don't remember anymore...
In gen 2 I got up to 250 Pokemon, the only one missing being Celebi.
In gen 3 I didn't complete it, but I caught everything that was catchable in Sapphire.
In gen 6 I was only 4 mons short in X, but I restarted that version after trading the important ones to my OR. At the same time I managed to get everything in OR, though most of the event mons where hacked, so they're not legit. At the same time, however, whenever I get access to a legit event mon, I like to release the hacked one, because why not.
Also, no matter what happens, I'm never going to complete a Pokedex ever again. It's just way too much work and the price you get for completing it, isn't even worth it, imo.