There are several options. I haven't' used them all. I am using a Wolf Eyes D26 XR-E Cree. It is one of the more expensive drop-ins. I bought mine from
Pacific Tactical Solutions. Make sure to also buy (check the box)) the
Spring for Surefire flashlight applications for it to work correctly in the G2. Mine works perfectly in all my 2-cell Surefires. Much brighter than a P60 and near the P61 bulbs. Runtime is twice as long as the stock bulb. Since the bulb is rated 3.7V to 13V you can also use it in your 3-cell Surefires. You can use it in a 2-cell Surefire light with one 16760 rechargable or two CR123's. You can use it in a 3-cell Surefire light with two 17500 rechargables or 3 CR123's.