haha lol that´s a good question lol. i think it really depends on your tuning. the lower the tuning the thicker the strings. that´s what i always do. but if your playing metal with a standard or drop D tuning you maybe should use strings which are thicker than normal(42-9 are ibanez standart i think). so use something like 46-10, 49-10.
If your using a lower tuning use thicker strings. with drop C i´m using 52s. for example
dude.. Piano Wires are the best strings for metal.. That's what all the pros use, dont cha know?

but seriously.. I Use 52-10 so 52,42,30,17,13,10
My Gear:
Gibson Flying V (active Emg's 81, 85)
Peavey 6505 Amp

Avatar Cab /w Vintage 30's / G12H-30's
Schecter Damien-6
ISP Decimator
Vox Wah Pedal
Ibanez TS808 Tube Screamer
Roland Cube 60