Do you have a budget in mind? Also what type of light are you after, as in flood, throw, long run time, short bursts of huge output??
I'm no expert on p60's, far far from it having only just got into p60's 2 weeks back.
Currently I'm running the Solarforce XM-L U2 3 mode drop in. Not sure on it's real ratings as Solarforce quote 500, 550 and 880 lumens for it
What I can say is, it's brighter and out throws my XP-G EagleTac D25LC2 and due to the XM-L it has a bigger hot spot and more spill.
For the money I reckon it's not bad at all.
However "International Outdoor Store" are offering a new drop in for $23.00 which looks very good. All are 3 mode and you can choose from 3 different drive currents and 3 different tints. PM me for a link if you can't find them via Google.
If you are after more 'premium' drop in's or multi emitter ones then I'm afraid I don't know.