First, I'm going to assume it is a current Surefire, and therefore LED. If it was an incan, 16340(RCR123) cells would be out due to the voltage difference. A CR123 cell is rated at 3.0v at a moderate current drain. A 16340 cell comes off the charger at 4.2v, and should be recharged when it gets down to between 2.5v and 3.0v - opinions vary. If it is a single cell light you can probably get away with using a 16340 cell. It gets more iffy with two cells. Instead of 6.0v, you start out with 8.4v, a significant difference. One can still find LiFePO4 cells, which come off the charger at 3.7v. Surefire used to sell those themselves, but now you have to find your own source.
I personally have a Surefire G2 and 6P with LED drop-ins, and use a single 16650 cell in each one.
If this is going to be your only need for Li-Ion cells, the Xtar VP2 would work well. It allows the user to chose between 3.7v, 4.2v, and 4.35v charging cycles. I am sure there are other good chargers out there that fellow CPFers will recommend as well. As far as cells, the rule around here is don't by anything that ends in -fire unless it starts with Sure-. Keeppower, Efest, Sanyo, Samsung, Olight, Nitecore are all good brands. In reality, there are only a couple of reputable Li-Ion manufacturers in the world, and the rest buy their cells from them and wrap them with their own logos and info, and many times add a protection circuit to help prevent overcharge, over discharge, and short circuits. There are a couple of good threads at the beginning of this Forum with more information. Member HKJ has the reputation of being the cell guru around here, and his website has reviews and ratings on almost every Li-Ion cell known to man. A very good resource when pondering cells.
A little ancient history here: Back at the dawn of time, there was the CR123A non-rechargeable cell. As the market matured and Li-Ion cells became more mainstream, a rechargeable version of the CR123 appeared, the RCR123 [RechargeableCR123]. As stated, things didn't quite match up, and the RCR123 was not ideal for many uses due to the extra voltage. As Li-Ion cells proliferated, they started becoming available in many different sizes. A standard way of stating the size of a cell was born. Using that standard, the CR123 form factor in rechargeable cells is known as the 16340/16350. Other popular sizes are the 18650, 18500 (two 18500 cells are roughly the same size as three 16340), 14500 (AA), and 10440 (AAA).