Alt-Right/Far-Right and it's not even debatable
Since '15-2016, we've seen them spew hateful, racist, and bigoted rhetoric and sometimes have acted out on it with violence, sometimes turning deadly, on top of looney conspiracies, and more often than not, they stick to their guns when it comes to spreading misinformation and half-truths no matter how ridiculous and far-fetch it sounds.
Many other things as well. America doesn't have a viable, let alone existent, Far-Left ideology or movement. What people are claiming to be Far-Left is just progressive and liberal perspectives, nothing that matches the extremities of the Far-Right/Alt-Right ideologies/movements.
People have been injured, killed, and/or become radicalized to harm and hurt others as the result of Far-Right/Alt-Right ideologies. Until some supposed “super duper” liberal goes on a killing spree or spreads misinformation, half-truths, and bigoted rhetoric, among other things, that get people injured or killed due to liberal/progressive rhetoric, the answer is the Alt-Right/Far-Right.