I always use Yahoo Messenger... It is easy to use and it gives all facilities, Like Voice Chat, Video chat etc...
I never had any luck with Trillium or the other "multi"-messengers. I've used a few, but Yahoo always seemed the easiest and most powerful for my uses. Love the photo-share feature.
i think YM/yahoo messenger is the best im chat software , cz it make we an easy to chat to others people .
If i have to choose from the options given. Then i will choose yahoo. Because i just use yahoo as my primary im. I used gtalk for sometime but yahoo is my primary im so i voted yahoo.