Should I focus on training a single champ in soloq?

First of all, I'm not a native english speaker. Sorry for my bad english. So, recently I keep being confused on what champ I should play. Everytime i play a ranked game, idk who to pick, just picking different famous champs like talon, kata. I once mained LEESIN but i found him really hard to carry. So, am i supposed to train a single champ? Or set a champ pool for myself? If i really want to train a specific champ as my main, which champ do u guys prefer? (He/She must be a tank or tanky dps, mid/top/jg, better be high influence in teamfight) Thank you :)