The three you mention are probably the main ones Aim being the AOL one MSN being the Microsoft one and ICQ being a largely independant service.
In each case you will need to be running the same program as those you wish to chat with so the answer is that is the only real critera to use.
Many of us regularly use all three and maybe also add in the Yahooo variety too.
There are also programs like "Trillian" which will allow you to combine the features of all of the above in one easy to use program.
btw IRC is alive and well but tends to be used mostly for specialist groups rather than general chat so you may have to do a little research to locate IRC servers that carry forums that will be of interest.
Yes there are a few problems with hacking and spamming but most can be overcome relatively easily. If ever a problem presents itself simply post about it here and I'm sure many will be able to help sort it.