Xerath/Velkoz/Cassiopeia ?

I am looking forward to buying one of these champions.The problem is I dont know quite well which champion would be good in solo queue and in general team fighting.I dont just want to be like a lane bully get bunch of kills in my lane and then do nothing late game.I would rather prefer to be carry and convert my kills in lane in Victory.So what do you think which champion should be good for the above requirements.
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