Mine was Magikarp/Gyarados.
He helped win so much battles during the Elite 4, so it was really worth it.
I actually caught it at level 5 when it was still a Magikarp but I gave him to the Daycare center and they leveled him up until 21. Then I used a rare candy to turn him into an almighty Gyarados. It was hard to level him up at the time because he was still weak and I was already at Celadon city. So I gave him the Water Pulse TM and kept grinding and grinding and grinding until he was on par with the other Pokemon in my party