I did catch all of them in FR/LG, and HG/SS. But my favorite would definitely have to be Articuno. I mean, just that thing.... it looks marvelous! Especially in Emerald and FR/LG, haha. And because it was Ice/Flying meant it could knock out most Dragon-Types ;; and in one hit because it's a legendary :)
Moltres also was my second favorite, quite close. The flames coming of its body look amazing! I really like his Pokédex entry too. It mentions something about like wherever it flies it sets ablaze. Now srsly, how awesome would that be~