Depends on what champs you play and how you play them however if you want to climb obviously mid is the best and adc+jungle are tied for second. If you really like support you can play it, but expect the climb to be hard unless you only play Blitzcrank. Solo q is all about how hard you can carry, your teammates really don't mean that much because you never know what could happen they might dc they might feed w/e so you just have to take it upon yourself to carry them. Also note if you're going to play mid and not pick a carry then you may as well have not picked mid in the first place (things like lulu mid or zilean mid is what I mean) I'm not saying YOU MUST PLAY MID just know that the carry roles are well.. easiest to "carry" on right? Jungle and adc are probably tied because they both kind of rely on teammates adc needs the support to be half decent, but can still solo carry late game. The junglers feed their teammates and have to rely on them to carry (for the most part, yes there are carry junglers.) After saying all this please please please don't play a role you don't like or don't have interest in don't play champions you don't like, my advice is generally what works for people who want to climb it's not for everyone. If anything pick you're favorite few champions and spam them get really good at them because it's much harder to play the game when you don't enjoy you're own champion. Good luck and just know that everyone's climb is different what gets me to diamond might only get you to gold k bai.