Mario and Diddy Kong are easy to pick up and a lot of fun. They're high on the tier list if you care. They play similarly as well, so if you don't care about one for whatever reason, try out the other. If you'd prefer to use a character who is a little more technical, try Zero Suit Samus. Just be aware that she's a glass cannon. Ike is also a fun character, but may rely on grabs. His f-air is excellent for spacing and zoning, and his b-air is probably his fastest aerial and excellent for KO'ing. I find his b-air satisfying.
Another fun Mario character is Bowser. He's faster than his previous incarnations, has strong kill moves, high durability. He just may be prone to being juggled, and his a rather large target. Characters with projectiles are pretty much safe, but from my experience, he is pretty damn fun. If you care more about the tier list, then perhaps Bowser isn't for you. He's ranked what I'd consider mid-tier, which isn't bad.
There are a lot of viable characters to choose from, so if rank isn't your concern, then what I'd suggest is trying out characters you find enjoyable to use. By that, I mean when you play the character and you feel like you're having fun, then go for it. Having fun can play a big role in terms of mental fortitude. All right, I might be bull****ting you on that, but I feel that it does help. Then break down your selection from there. Perhaps there's a character from one of your favorite series. Try that.
The important thing is to have fun with your character. Some people don't play as certain characters, even if they're top-tier, because they just find those characters boring. For me, it was an easy choice. Metroid is my favorite fiction of all fictions. So I decided to choose Samus. I'm trying to learn how to use Zamus, but sometimes if she's not working, I have to bring out the big guns, which can be ironic, since I perform so much better with a low-tier than a top-tier.