I have mained a few characters in my time. I will start with the order of choice.
Ike: Ultimate four player brawler, so amazing at racking up kills and interrupting quarreling groups by just a flick of the c-stick. Plus he is like one of the only characters who can KO easily with a simple A combo. XD
Captain Olimar: Simply fell in love prior to the games release, wanting him to be in the game above any other character, and I was so excited he was in that there was simply no way when I delved into his character that I would be disappointed. He does have nice, fast smash attacks and fun ariels.
Lucas: A very quirky character. Never liked Ness till I found Lucas, his resourceful blend of powerful smash attacks (powerful up smash, double striking down smash, fast forward smash) and intricate special attacks that cover distant foes.