We buy most of our houses at the courthouse steps. In spite of that I'd say that it is not at all the easiest path. It is probably the simplest transaction - if you are the high bidder you immediately hand the trustee cashiers checks for the full amount, and then Texas at least you typically get the trustee deed a few weeks later. Minimal paperwork and no closing costs. It does however take a tremendous amount of research and entails a larger amount of risk. There is no option period, title insurance, etc so it's often a good place to start.
In most cases I find that individual REO properties are a much easier place to start. In my experience they have generally been straightforward transactions. You make the offer, wait on the counter and if it's accepted then you close in a standard fashion at a title company. Basically a numbers game. Some banks are slow to respond and most are fairly inflexible, but if you write clean offers that don't need closing costs and don't want exceptions then you will rarely have a problem.
The biggest problem I see with REOs is that that are too easy. They are listed on the MLS and everyone has the same access to them. Because there are no real barriers to entry, there are a lot of offers on anything that is a good deal. In most other cases the banks have it priced too high for it to be a good deal. The key to anything you want to flip is buying it cheap enough to have a margin for error and it's hard to find REOs that fit that criteria, at least in the Austin market. We've purchased a few so far this year but it can be tough to build a consistent volume just doing reos. It's definitely a nice supplement to the rest of the business though.
As far as rankings from hard to eas, the best answer is probably that it depends. It's really an issue of doing enough deals that fit your skillset. Some people love marketing and talking to home owners. For them preforeclores are probably a good fit. Other people tend to like research and pursue the courthouse steps. Others like neogitations and don't mind paperwork and do well in short sales. The key to all of it is probably persistence - deals that are really good enough to make money on are out there, but they take a lot of work to find.