Forehand. More specifically my down the line/off-forehand are my best.
On a good day my serve is very good aswell. Serving balls at one of those speed things at the Aussie open I clocked 155km/h, and that was with only 4 serves, and not having picked up a racquet for 1 1/2 months before, plus apparently the speed testers understate your serve. Plus I was only 16 at the time. But on my off days, i'll struggle to get one in.