The oneway chuck is great; I haven't used mine nearly enough. I actually got the jumbo jaws with it as well, but have never yet had the chance to try them out. I am still excited about just being able to make candlestick bases that don't have faceplate screw holes in the bottom! If I did more turning, I might nominate this accessory as my 2nd choice, but since lately I have been using the SS mostly in the tablesaw mode, I will restrict my choice to SS accessories for the saw. Therefore, I would say that my 2nd favorite accessory is...the envelope please...
The sliding crosscut table.
Before I had the SS version, I had been using a couple of shopbuilt sleds. They worked fine, but were smaller than the SS accessory and sat on the saw table, straddling the blade. The SS sled is offset to one side of the blade, which for the SS table is a better design. Also, I really appreciate the side to side size of the SS table that results from this offset design. I am using it on my old 500 and it really increases the table saw work surface. If I have wide pieces to crosscut I add a front table extension to the main table and another to the extension table and gain quite a bit of width capacity that way. The cross-cut table comes with pre-installed threaded inserts so that the fence can be angled at various angles (22.5, 30 & 45, as I recall), so the table can take the place of the miter gage for these common cross cuts. I haven't tried them out yet, but I can say that once the fence was adjusted initially at 90 to the blade it has stayed put and has made for really accurate cuts.
Come to think of it, I might nominate the front extension tables ( I have 2) as my 3rd favorite accessory. They are so easy and quick to attach that I find myself using them quite often in conjunction with the cross cut table.