Johto is by far the favourite, how did I guess...
Here we go~. There's lakes, a particularly Japanese styled city, large park, equally large forest, wonderful mountain roads... and a dragon dark Unown ice cave! :D Just to name a few things the region has to offer.
Plus I reckon Goldenrod is the largest and most detailed city they ever bothered to create, with all those house sprites... lol. Oh, and the underground there too...
Lest we forget... Tōjō Falls! :D ... Though that is technically in Kanto, it's only depicted in the second generation games.
There's something about C/G/S's graphics that appealed to me aswell... though that's not related to the region alone, but still...