I want to create my own IM and I'm searching an open-source IM APIs. What do you think is the best open-source IM APIs. And what good front end to use?
If you are looking into making a client, check out libpurple. This is what pidgin and many other IM clients use to access multiple IM networks.
http://developer.pidgin.im/wiki/WhatIsLibpurple
If you are just worried about one IM network, the easiest one to work with would be Jabber because it is an open sourced protocol