The most FUN? Honda CRF230F. Turns like it’s on rails and is mild enough to ride it for all it’s worth and can ride in snotty conditions or rainy days when more serious bikes are unridable or no fun. Just don’t jump it. Most fun “serious” bike I’ve ridden is a 2006 Suzuki RM250. I describe it as a fast, torquey 125 and is a great jumper and is tractable enough to ride fairly slow and technical too.