Liberal vs Conservative is quite cool, if you're referring to things like the Gun Laws, the election and stuff, the debates usually get heated but people still put their points across even midst some insult throwing. I'm not well versed on the American situation so I don't usually take part but they are fun to read and I can usually see the argument being made for both sides even though I am more Liberal myself.
Racists vs Non Racists, Conspiracy vs Non Conspiracy and Atheists vs Religious people are always too one sided to be anything interesting.
Mayweather vs Pacquiao is really just people insulting each other these days.
Mexico vs Puerto Rico is hilarious as they are usually somewhat civilized and then someone will post something like "All Puerto Ricans are f@gs who take knees" or something along those lines and all hell breaks loose.