PS2. It is the console which I have the most games for. After that I'd say the DS, which ranks in 2nd in terms of amount of games I have. The PS2 is my favourite home-console and the DS my favourite portable one.
NDS - There are many good games which I've enjoyed on this console, and most importantly, a very big thank you to it for accompanying me on my travels to my destinations. It makes the trip seem so much shorter!