Celluloid Picks made by Fender and many other companies are standard. They're fine but the thin ones break easily.
I have a slight preference for Dunlop Ultex .73 picks when playing an acoustic. They provide a little more friction when striking the string, creating a pleasing chime sound. I haven't used the lighter ones but I'll bet they're more durable than Celluloid.
My favorite pick for playing electric is the Dunlop Nylon Standard .88 pick. It has a grippy surface and won't tear like the celluloid picks.
If you struggle with holding on to your pick, Cat Tongue Picks are the grippiest I've found.