Probably Comcast but you'd need to test to know for sure. Traceroute is your friend here. If you open a command prompt and traceroute (or tracert on Windows) 216.52.241.254 then you should be able to see where your packets are going and what jumps are taking the longest.
It takes some work but if things are working correctly each hop should have a low latency time and they should generally be headed towards Riot's servers.
The first problem is easy to see. If one hop is taking a long time that's probably where the lag is coming from. If that hop is somewhere in Comcast's network then it's their fault. If it's somewhere close to the end of the line then the problem is Riot or their ISP.
The second problem can take more work to suss out but if you look up the internet providers your data is passing through you can figure out if your packets are taking a wrong turn somewhere. I have yet to see this problem caused by anyone but Comcast but there's always room for new things in the world.