I rarely find myself offended by someone's political beliefs in a very visceral way, buta little known poster named Koenkai managed to trigger me something awful. If there was ever a poster on this forum I found myself in persistent, loud disagreement with, it's him.
Koenkai was definitely a very interesting poster. I'd forgotten about him.
Least Similar is very difficult; my mixture of positions is very eclectic and has only gotten more so over the past year and a half. There's no one on the forum who I don't have an issue of agreement with, somewhere. I'll say one of our Marxist far-leftists, like TNF or Snowstalker, is least similar to me, but there might be a better answer out there.
Most Similar is almost certainly Goldwater. I think my most persistent disagreement with Goldwater has been about weather.