MSN Messenger - 6.0
Yahoo IM - no idea what version.
AOL Messenger - ? version??

I started out with Yahoo. Most of my contacts are on yahoo IM.
The members of my Girl Scout troop mainly use AOL. So I installed AOL IM to chat with them. Besides troop members, a few leaders & 3 or 4 family, I don't use AOL much.
Quite a few friends prefer MSN and only use that. So it was installed next.

I probably use Yahoo and MSN about the same amount of time. They both have their good & bad points. I think I like Yahoo best just because of the way it's set up and ease of use. Both Yahoo & MSN give me trouble from time to time. So both are unreliable. All personnel preference I guess.