You're not going to consistently get 20 kills per game on any champion, no matter how skilled you are with it. There are some champions where it's more likely that you'll be able to get 20 kills some of the time (like once every 20 games), but none where it's anywhere near common.
You can check out http://champion.gg/statistics to get a basic idea of how many kills champions in different roles will get. Note that the highest top laner is only averaging about 8 kills per game, and the highest mid laner is getting about 9.5.
Talon seems to be high in both roles, and he also has the benefit of being able to escape after getting kills if there's nearby terrain, which is not super common for assassins.
But as Chroma Spell said, focusing on finding a champion you like is most important. If you're willing to spend real life money to purchase a lot of champions, it's not such an issue. But most people get a limited amount of IP. So you should try out all the champions that are available for free each week, and hopefully one will click with you :)