The 2009 world champions told BBC Sport: "I really don't know who's quick. But the one team that has been consistently quick has been Mercedes more than anyone else.
"I like driving [the 2013 McLaren]. It is a nice car to drive. There is nothing that scares me with it but I still feel we can do a lot more with it than we are at the moment, and that's the exciting part, that we're not at the end."
Button finished the second day of the final winter test in Barcelona with the second-fastest lap time, while the Mercedes was 10th.