Aside from certain characters being strangely powerful (which is why I veered far away from playing Diddy and a few others), Smash 4 feels pretty well-balanced aside from the occasional counter situation where some characters seriously struggle with a few specific ones (i.e. projectile users against opponents with good movesets to combat projectile usage, like Fox and Rosalina), but that's usually inevitable.
Funny enough, I have no freaking clue where Dr. Mario stands on tier lists, and he's been one of the most reliable characters for me zel. The others I often use - Falcon, Link, and MK - all seem just fine in most battles too. I can't really called anyone OP in this game barring occasional certain matchups (and pre-nerf Diddy), only a few feel a bit underpowered (WFT and Swordfighter).