KingG8rI bought Panasonic NCR18650B, however I think their max amperage is limited so you might not be able to get full brightness with them. I think the only requirement you really need is that it's an unprotected cell, and Nitecores come in both protected and unprotected varieties. I'd probably buy LG HG2 18650s if I were to buy some more.
KingG8rYou need a high amp unprotected cell. I believe technically you need the cell to support 8.something amps of draw. So go with a decent 10A or greater cell. It needs to be a flat top cell as well or it won't fit right.
For a cell that won't leave you hanging if you get the next generation of pocket rocket, the Sony VTC6 30A cell is currently king of the heap and has more discharge capability than most lights will ask of it.
Samsung 30Q 15A cells are very popular. I'm using LG Mj1 3500mah 10A cells for the extra capacity, and they work fine.