I would eliminate the O1 as an easy steel because of price and the fact that the time/temp. chart for heat treating O1 is about 10 seconds. A temper line is very hard to do on O1. For the clay temper, the 10XX, 5160, and 52100 are all good blades for a temper line. One thing I have learned though is to make sure that the steel has been normalized at least twice and then annealed. If the steel is hard to start with, it will be hard to show the temper line. For the annealing, I submerge in lime, vermiculite, or wood ashes. An added precaution would be to stick a piece of hot steel in the medium first to warm it up before you put the blade in. Let it cool to the touch and it is ready to drill and grind. Before you apply the clay, mix a light slurry of the clay and coat the whole blade (a verry, verry light coat). After it dries, apply a light coat (1/8" to 3/16") and let dry. Apply the clay until there is about 3/8" of dried clay in the area that you want to remain soft. A scalloped or uneven edge looks better to me but I don't usually get it unless I really try. Some bladesmiths will draw the back while edge is submerged in water to make sure it is soft. I check it with a file and if the blade was annealed, it will be cut easily with a file. I read at one time that it is best to quench on the rising temp. If you set your oven at about 1550 deg, and take it out the moment it reaches that temp., the edge will be getting hot before the clay covered steel and the hot clay will cause the covered steel to cool slow enough to remain soft. The light coating on the blade will help the blade cool faster by not having a layer of boiling gases to insulate it.
Didn't really mean to write a book but I couldn't say part without the rest.
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Ray Kirk
www.tah-usa.net/raker
The main reason I'm getting away from stainless is an interest in differential hardening and possibly clay coating and water quenching blades. So, given an interest in temper lines, what is the cheapest/easiest/hardest to screw up steel to start with? I'm kinda leaning to either 5160 or 1084 or maybe O1. I have access to a HT oven, but would prefer to do the HT from the forge, with a minimum of stuff, so I can do it at home. 
