I pick my main champions over counter picks. Now, if the matchup is extremely difficult, I won't choose my main that gets dumpstered unless you are THAT confident. For example, I used to main Syndra when she wasn't so buggy and if the enemy team has a yasuo mid, this is a tough matchup for Syndra, I wouldn't go into that matchup. The sheer mobility is difficult, let alone his wind wall to cancel Syndra's ult. However, I decided to be cheeky a couple times and picked Syndra anyway despite the disadvantage I had only because I was very confident in my skills. I ended up defeating him (and other yasuos) with some ease. In any case though, I would choose someone that doesn't get super countered but also someone I am decent/good with. That is why you pick up 3 champions in a role to master, hopefully 3 champions that are different from each other for the most part.
NEVER pick a champion because they're considered "OP." Especially if you don't know how to play them or barely know their kit and limits.