Pokemon Ruby. It was my first GBA game (along with HP2: Chamber of Secrets), and I've always liked the Pokemon and stuff found in it more than in Sapphire. But Pokemon Emerald tops both <3
Third games in the various series or first two? (in other words, Yellow, Crystal, Emerald, Platinum, and Explorers of Sky versus Red and Green/Blue, Gold and Silver, Ruby and Sapphire, Diamond and Pearl, and Explorers of Darkness and Explorers of Time)