Firstly, any champ will be strong if they're fed.
One thing to say is that you can't bundle champions like this. People like to apply the labels "Bruiser" "Juggernaut" etc. and those labels reflect the most optimal way to play a champion that players currently know of , but at the same time the AD Assassins are best dealt with in different ways, the mobile bruisers are best dealt with in different ways. The labels don't mean much. You should consider each champion individually.
For instance into Rengar I would pick Alistar, because Alistar can Q Rengar out of his ultimate's jump, and Headbutt the Rengar away afterwards if the Rengar isn't dead already. However into Zed, another AD assassin, I find a lot of success with exhaust Soraka because I can Silence his W or R shadow when he comes through them in a team fight, I can keep allies high hp and out of his non ult lethal damage too, and if he chooses me as his primary target, I can silence his R shadow behind me so that he dies to kill the support. Alistar isn't as good into Zed because Zed can R in reaction to your Q, W in reaction to your combo etc.
You see? you've got to think about how to play around those individual champions with your champion and then put that into practice and see if it works