For me it was Articuno. i caught moltres on my first try. Caught Zapdos after about 2 tries with a great ball, i may add. But Articuno was just stubborn. it took me like 9 or 10 tries just to get it. Plus it's life was red and it was paralyzed.
Seriously? Wow, it took my ages on LG, but in FR I just ordered a Drill Peck from Zapdos and a Blast Burn from Charizard and one ultra ball means Moltres is caught. I dropped my GBA, and the game reset. I done exactly the same thing and I caught him again. To be honest, Moltres was pretty hard for me before I did that.
I got a really weird thing...I timed my catches and i got almost exactly 17 minutes for each one...all you have to do is get its health down and either freeze, paralyze, or put it 2 sleep. I like paralyze cuz it doesnt wear off unlike all the others.
Moltres (even if it was my fave) was a big pain. I had to keep soft resetting so that it would use Endure, after which I woul use Blastoise's Hydro Cannon or something that would faint it. Then came Thunder Wave, and it still took me way too many times to catch it. It had terrible natures each time I caught it, too...