1 - Every role has the potential of influencing the game enough to be considered "hard carrying" except for support.
2 - Play whichever role you feel you're best at
BUT
3- whichever role you pick, take advantage of easy champions.
Tip 3 differs based on your current and desired elo but I'd say having a mastery over extremely basic champions will get you up to plat relatively easy when compared to being "just ok" or "pretty good" at a skill intensive champ like yasuo.
4- the lower the elo, the longer games tend to be. Especially in silver and below, late game hyper scaling champs have the advantage since teams tend not to know how to and a game quickly.
Top lane - juggernauts and splitpushers like garen mordekaiser, tryndamere are very easy to play with the right mindset.
Mid lane - Annie is extremely powerful with a very easy game plan (flash ult). Veigar and malzahar are also good. Mages in general tend to be easier and more accessible than assassins.
Jungle - it's your choice between gankers and power farmers but keep in mind that simplicity and scaling are your friends. Vi, udyr, Warwick, master yi are all great.
Adc- simple but effective ones are jinx, Caitlyn, and kaisa if you have good aim.