Emerald all the way. Partially because yes, it is the first game I had too, so there are happy memories, including the fact that I couldn't complete it until late 2006. And it's the only game where I've ever racked up 100+ hours on it. But also because unlike LeafGreen, my other Gen III game, it has Battle Frontier, Contests, Gym Leader re-matches and quite a few legendary pokémon to capture. It also includes a lot of my favourite pokémon in the wild, even ones that I like but would never use.
Don't get me wrong, LeafGreen's good. It's just got a bit less to do once you defeat the Elite Four (although the Elite Four re-match does help a lot with training, and give you something to train for). It also has a lot of my favourite pokémon in it...but I always struggle for a team pre-Mt. Moon. Once I get there, I can get Nidoran, Clefairy...it just opens up a lot more and I'm happy.
Whereas in Emerald, there's such variation before the first gym - you get Poochyena, Zigzagoon, Wurmple, Ralts, Seedot, Lotad, Wingull, Marill, Shroomish, Nincada...that's nine types just from the pokémon I can remember. In LeafGreen, including that one leading to the Indigo Plateau, you can get Rattata, Pidgey, Spearow, Mankey, Pikachu, Weedle / Caterpie and evolutions - 7 types in four areas.
So whilst I love both of them, Emerald is just so much better than LeafGreen, even removing the "it was my first game" factor. Although I may be biased - look at my theme :P