Guitar Strings

If you're playing in lower tunings, you're probably going to want to use thicker strings (gauge 10s or 11s would probably suit you well). I use D'Addario 9s, but I play in standard tuning.
 
Yup. D'Addario 10's would be perfect. I use 9's and I play in Eb. Perfect tension for me.
 
D'Addario regular light gauge(.010 to .046). The thicker your strings are the more stress on your fingers and hand. If you want to skim the neck faster and more clean don't get some big stupid strings just because you want to drop tune, regulars sound fine in lower tuning.

D' Addario sound better at first and break in faster than other strings imo so if you have to practive with people you will not get completly fucked over. Bigger strings sound the worse when they're not streched or broken in.
 
http://www.ernieball.com/site/products_strings_nickel.php

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Depends on what you play
Since I have a grungy tone turned metal, 11's work for me at D or drop C or even drop D or standard...
There are many sets on the link I sent you. You simply have to choose what you want.
What tuning do you normally play in and what do you want your sound to be?
(describe it please, as if you name a band I may not know who the fuck you are talking about)