I'm certain the closest you'll get to a very high damage assassin that can hold their own in a 1v1 situation is Akali. She has a stealth of her own, and her burst is unbelievable as well as having a fairly low skill cap. Otherwise, I hear Kha'Zix is very strong. ^^ Hope this helps
Edit: Also, ganking isn't only for your benefit of getting kills but it will help a lane that was in trouble of being zoned or lost, which will hopefully allow them to catch up and give a higher chance to win the game. As mid, I gank both lanes pretty often and sometimes even help the jungler to steal red or blue.