the only really good picks are the ones you like to play with. depending on the guitar store (some get mad at this) grab a few different ones and play a similar guitar to the one you play. if you dont like any of them go back for some different ones.

if the store wont let you mix and match like this just go and buy one of each. picks usually arent more than 50 cents a peice or so. find the one or two you like then buy more.

as a point ill give some info on some types of picks. the thinner a pick is the more it will bend as you hit the string, so light picks are good for novice strumming but they are harder to play single note passages with because the bends too much, medium are right inbetweeen and are good picks all around. heavy picks are hard to strum with but they dont bend so play fast riffs (like metal) is a lot easier.

as a point of reference i use the dunlop purple nylon ppicks at 1.5 mm and the dunlop big strudy at 2.0 mm. they take alot of practice to be able to strum with but for playing solo and picking patterns theres nothing better for me. and i shopped online and got 6 dozen of each so i wont need to buy picks for a while.

but you need to find the picks that work best for you, and that will change as you get better.