I cannot stand Ted. I find him whiny and just annoying. Marshal is just so cute and compliments Lilly ballsy but I look innocent look so well. And Barney? Well... I have no idea how they have kept his character so entertaining for so long and it's great to see how he has grown and I love the twisty episodes when his friends think he is not really clever but he shocks them all.
Robin on the other hand... she is a much needed character, but I find her a little grating at times. I like this last season the most where she kind of got some spunk and started to take life by the horns a little.
So I cannot pick just one as they compliment each other.