This was actually really hard. I'm using such a wide variety of characters in this game because I honestly feel like they've perfected nearly every character. Diddy, Ness, Sheik, Duck Hunt, Cpt. Falcon, Zelda, Robin, Shulk, and Peach are all characters I switch back and forth from, but I'm almost unstoppable with Ness because I have the PK Thunder Self-Projectile down to a science. It's an insane threat to others, and no other Ness I've played against online (I've played about 500 online matches so far) has even got the Thunder projectile itself down. You gotta make them think that you rely on the self-projecting and range, then they get in close and surprise them with a knowledge of his close-up moves.
But Diddy's air game is just fantastic. I love how fast and snappy his air moves are and he's such a tricky character to play against that I feel like I piss people off sometimes.
Other than that, and despite the ridiculous amount of time I spend with this game, I still haven't really tried a lot of the characters. I know that Bowser is high on my list of characters to try out next.