In terms of schooling part yes this is true, in terms of the social life outside of school I think you'd have more time in M1 and 2 to do things you like, such as my friends that go to the salsa or swing dance clubs, or the people who want to go to bars and go drinking when they have free time, or the people who want to do other outside things. And some newer curricula like at FIU and UCF are incorporating this thing in their academic curricula where you get involved with something that you can work on a project on that does not necessarily have to be related to medicine so it can be research or it can be something related to teaching or music, etc.
So in that aspect it could be some what fun.
But it depends on what we are talking about again? The schooling aspect or the social aspect to determine what years are more fun. And also what curricula are just pure hard science and which ones incorporate mini concentrations or these independent projects.