It'd be different depending on the country. For the UK though, I'll give you my thoughts. I pick padlocks now and again but most of my experience is with door cylinders, so I'll talk about those.
Easy would be anything with standard pins and average/below average tolerances. For cylinders, try:
- Cheapo brands such as Bird, or anything with an obscure name
- Cylinders with no name whatsoever
- Most ERA rim cylinders, and non-BS rated euro profiles
- Older models of Union
Medium would be locks with security pins and average tolerances such as:
- Iseo
- Tesa
- Basta
- Asec
- Zone
Moderately difficult stuff would be locks with security pins and fairly tight tolerances, aswell as trickier keyways:
- ABUS
- Kitemarked Yales (the mortise cylinders have much tighter tolerances than the euro profiles for some reason, I'd recommend trying the euros first)
- Kitemarked Unions
- Corbin
- Cisa (some of these have trap pins and use spooled key pins so keep this in mind when purchasing one to practice on)
That's plenty to get you started. Once you get onto higher quality stuff you won't need a list of stuff to go by, cos you'll have made your own by then. You won't be looking at anything harder than that last list for quite some time, believe me.
Hope this helps, it's not an exhaustive list by any means and I'm certain I've left out a whole bunch of stuff, but it'll get the ball rolling anyway.