Nico Nico Nii!~
Before you answer, it can be for just about any reason, so long as it's not just "because it's the best" or if the reason is not PG-13! You can choose more than one set, too, if you're having trouble picking just one!
Personally, my favorite set is the HeartGold SoulSilver Base Set, simply because of the card borders.
After that, it's the Japanese Shiny Collection because of its full-art cards.
So, what are your favorite sets and why?