I agree with Brian answer but I want to add a point.
You saw the summer NA LCS?
Cloud 9 was the best team by far, and they always pick early/mid champs.
Why? It's better to take early champs? Not always.
But if you take early champs you need to know how to play good at lane or do a lot of teamwork for get fast objectives and get early gold to get better than the other team even in lategame.
If you are playing solo/duo and you like to play mid (thats my main) I advise to you to take mid champs like kassadin/Diana/Akali/Lissandra/ahri if your bot is late like vayne. Cause if you see your vayne is 20 cs less or 0/2/0 you can roam fast to help her to get feed.
But like brian said: "You should try to get a team comp that synergizes well." Like taking orianna if zac or j4 in your team.
And another advise. If you dont see any great pick to do a great teamplay, take a champ that can split push really well like tryndamere or Yi and push one lane all you can. In my experience, that can win a game. (e.g. See fnatic games).
Sorry for my bad english.