Litwick: "Haiz! I Litwick, and I steal your soul! And I'm cute to fool you, so can I eat your soul? You won't be dead, because I'm dead! Your dead is my alive! See what happens when I blow out my fire. *freezes and becomes candle, then lights back up* That's what happens!And then I steal your life force because I like my sister Lampent! And cookies are great and what are we talking about?"
:3 I has it!
Clickee to go to the Rulers of the Universe.
Original Game: Still haven't beat it.. I'm now ashamed of myself, it came out years ago
Shadows of Almia: Drapion, by far. That think made me rage even more than I did when I battled Whitney's Miltank. Second is Dialga, knowing that at anytime, Dialga could just end the capture. Regigigas was also very hard.
Guardian Signs: Mewtwo with the purple stuff around it. Very hard to capture with the stuff around it. The Ditto capture was very unique, as it could transform into Raikou, Entei or Suicune.
It's been so long since I've played a Rangers game. Guardian Signs just left a horrible taste in my mouth that I haven't washed out yet...
From recollection, though, the boss that I've struggled the most with was Charizard in the original. That battle single-handedly honed my skills with the Capture Styler, and every boss afterwards was a walk in the park comparatively.
Mew was the hardest in the original Games. It could just flee and required quite a few loops to capture!
The Final Room in the Capture Challenge was the home of Charizard, Flygon and Salamemce. Flygon was really fast, Salamence, while being slow had really powerful Moves at its disposal and Charizard seemed to be the most balanced.
Lugia seemed to be the hardest to capture in these Games. It was fast and could leave 'traps' for us to deal with.