The only games are in the third generation really. No Pokemon that are between 151 and 251, and no Wi Fi. Also, no backwards compatibility between G/S/C, so it's the only one.
As said above, all the Gen 3 games by far, especially since the Mew event was so limited. The international versions of the Gen 2 games are an honourable mention as well, since iirc, the Celebi event was exclusive to Japan.
Well, it won't actually be 721 when I transfer it since I traded away shitty fodder Pokemon I dislike and a lot are in their fully evolved forms without their pre-evolved versions in the box, but yeah I'll transfer. Would be a huge waste if I didn't.
Not to mention that there's no need for mystery gifts for completion in gen 1, unlike the rest (unless barring Celebi from Gen 2).
As far as finances go, it also helps that the Gen 1 games are the cheapest when comparing their virtual releases to any unused games of other generations.
I don't think completing the Pokédex is that big of an achievement, outside of bragging rights against whoever. Mostly from what I see, people play for the in-game story, see a new region, than spend the rest of that era EV Training/IV Breeding for the competitive scene.