What is most important in picking a main champ for ranked?
I would say the the most important thing for being succesfull is for the champion to be at least decent, and you must have experience on that champion. EX:Having experience on
Urgot won't really make you win games that easily. Especially since your team will flame you for picking a "troll pick".

Know the strengths of champions you pick, and take that to advantage. However, using some champs will eat away at your skill on other champs, and vice versa. I used to be very good with Wukong but after switching to things like Renek and Volibear, it just wasn't the same. I usually get two tanky items on Quinn because it's covered by her attack speed bonus in ult and when she's ranged you can proc your passive multiple times making up for the damage gone from building two tank items. I guess this is just a playstyle thing rather than talking about ranked though. But yeah, playing champs you like with knowing their strengths and weaknesses helps a lot.
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I would also recommend to build experience on champions that are not those which people have used to be playing against. I have noticed myself that enemy players usually tend to have trouble even playing against rare builds etc.
A few examples of my own:
Cassiopeia
Shyvana
Ashe
I hope this was helpful :3