Gen 2 - Ampharos for sure. Getting mareep ASAP in G/S then boxing my starter using Ampharos/Flaffy/Mareep for the one Pokemon challenge. Thunderpunching my way through the Elite 4 and over Red. Then to have it broken with the next generation of Physical and Special moves instead of elements being the factor. Totally trashed my Thunderpunch machine. =(
Gen 3 - Blaziken, my savior in Ruby/Sapphire, learning about EVs and IV's, discovering Earthquake was uber destructive. Developing first move set, Blaze Kick, Sky Uppercut, Earthquake, Strength (WTF?! in retro)
Gen 4 - Empoleon, the cute surfing penguin with bladefins for arms, Diamond's one Pokemon challenge champion. Lucario, my first AR pokemon (gave it Ice Punch) ony to have it move tutored in PtHGSS.
Gen 5 - Excadrill, the ground Empoleon(how I saw it anayway lol), with 400+ Atk and STAB Earthquake. I grinned so much when I realized what a powerhouse this would be for the one Pokemon challenge. Victini the Fire Mew which can be cloned for Atk & Special exclusive movesets. (Still waiting to receive my Victini with Bolt Strike =) )
I still can't use pokemon "teams" till metagame since I read about the one pokemon challenge in G/S... Just seems too easy.