(Question taken from If x2 by Evelyn McFarlane & James Saywell)
If I had to take away one emotion from my life, I think I would chose to get rid of regret. Whether it's regretting not doing something, or regretting doing something; it's a terrible thing to feel. If you could eliminate that 'what if' feeling, wouldn't you? No matter what you do, you would never wonder if you should or shouldn't have done something. You could live your life not dwelling on the past, not thinking about that opportunity you passed up. In so many ways it would make your lifestyle better, the past is the past and regret is a big part of why people can't move on from it.
Although, it would have a negative side. It may mess with your moral compass a little, if you did something to hurt somebody and you didn't feel regret about it? You didn't regret hurting a person.. that would be a negative side. However, every emotion you pick will have its negative side so my answer would have to be regret.