MSN 6
I downloaded MSN 6 yesterday and aside from some issues signing in, it seems to work great.
My only gripe is that they still have not added my favorite ICQ feature; the ability set your status (offline, busy, available, etc) for each person individually. I used this feature a lot in ICQ so that I wouldn't get interrupted by friends and family while working. But I also want to remain online so I can be seen by the rest of my team who do need to be able to interrupt me on occasion.
Without this feature, as soon as I go online I get assaulted by 100 IMs from everyone I've ever met in my life. :-)