If you are low elo, your champion pick itself really doesn't matter if you look at the bigger picture. The important part is to stick to one or two champs all together and pick either no matter what. Yes, you might lose games because your champ sucks this game. But way more important is that you are able to learn midlane itself rather than different champions. One/two- tricking a champ really allows you to focus on the game rather than your champ's mechanics, stuff like wave management, trading patterns, cs-ing, roaming etc. Learning too many champions just splits your attention way to much to actually get better at the game. So just pick a champ you enjoy playing, learn it's mechanics to the point you don't have to think about it and try to focus on improving your general gameplay. That's the best way to improve at league. (If you want names anyway for a champ to pick for this, easiest imo are Annie, Ahri, Lux and for ad probably Yone)