I used to ladder a bit and experiment with new teams during the first PCL, but while I had some success (lower ladder of course), I was only truly confident with just one specific team, which I took from one of Anti's RMT threads and made some changes with Pokémon I enjoy using (Scarf Lando <3) and then a few others to take care of some threats.
I don't think I'm a great battler but I think given enough time and practice, anyone can be a decent battler once they get to know their teams' strength and weaknesses. And even if you're not great at team building, you'll probably be forced to make changes for the better after some practice if you notice your team does terribly against specific threats.
At this point I've definitely not been battling enough to be confident with any team, though I'd like to work on that once I'm done with exams. I do REALLY bad against stall teams though, throw in a Chansey and Stealth Rock in your team and I'm probably going to lose. I need to work on predictions and double switching, at least against those types of teams.
I've not played SM much but I know the OU tier well enough, though I'd really like to get back into LC.