I find the Brinks diskus (its flat and round, looks like a donut) lock (available at my local Walmart and Ace hardware has them too under the Ace brand) is a little tougher than a Master and the pins are in the bottom of the keyway, which is a switch for most North American lock pickers (that is common in Europe, I hear). Also, unlike most padlock, it doesn't have a spring so it more like a deadbolt in 'feel'.
You can also get the Brinks House Key Compatible padlock. It has spools but is really pretty easy (just use lots of tension, it has a killer spring). The good thing is that you can repin it (they give you the pins) to make it harder to pick (you can place short pins next to long pins to increase the difficulty). Its a good introduction to spool drivers without being too hard.
The toughest locks Walmart sells is the Brinks Shielded Shackle (it has two rectangular shields that protrude up the sides of the shackle) and the Master Platinum series. The Brinks Shielded has serrated pins and the Master has a small keyway, pins in the bottom and mushrooms or spool drivers (not sure).
Gee, can you tell I shop at Walmart alot?
