I love Gen 3. I started there (Sapphire), It introduced (at least to me (Skarmory), as I never played all the way through a gen 2 game) a lot of my favorite Pokemon. I spent like 600 hours on my original file completing the regional dex, and my starter's descendant (IV bred Sceptile) is kicking but online. I also love the variety that fills the region, and despite the Tentacool and Wingull, I love the Yin-Yang looking balance of land to water in the region's map. The sound was amazing, especially the well placed piano pieces scattered about in just the right spots. I also liked Leaf Green, but I got that after my friends and I stopped playing Pokemon as much together, so it hasn't really stuck with me as much.
TL; DR: Hoenn in Gen 3 was amazing! Always gonna be lovin' Treecko!
PS: Alpha Sapphire is an amazing remake, but they made the game too easy all-round for it to measure up. I don't like what they did to the exp share, played without it 90% of the time.