I liked Pokemon Emerald the most. By the time I was introduced to Pokemon the DS was coming out so everyone used GameBoy Advances, so I was given a GameBoy SP and Emerald for free by a mate. I went on to play Diamond, Pearl, Link, LeafGreen and Sapphire, but I just didn't like any of them as much of Emerald. The old ones lacked something I couldn't put my finger on, and the new ones have some really stupid Pokemon with no characters at all.
Since I started playing Pokemon on the GBA: SP I never really played the older ones. I now use my Gameboy Micro if I ever need to play GBA games, and they can't hold older cartridges, so I still cannot play them.
I was ADDICTED to that game! I would play for hours
On Emerald I got a shiny Medichan. Shiny Pokemon are Pokemon that are not their usual colour, and as far as I remember there's a 1/8000 chance of finding one. My Medichan was blue instead of pink!
And Piplup was alright. I liked fire and rock a lot more though, it did make ice and water Pokemon very difficult to beat.