Kingnog, "best" has to do with a number of factors -- not just what the flashlight is rated for, but other factors could include how responsible you will be in maintaining your cells, whether the light has built-in over-discharge circuitry, how much current the light draws on high. In other words, if you really want to learn more, it might help to ask this question in the context of a particular light.
I can tell you what we do know already. For a light that can handle 4.2V, we can look at HKJ's chart in the 2nd post and know that we'd go with ICR or IMR (we might as well drive the light at the max voltage supported, unless we know that the light is tuned for a different voltage, the way some zebralights seem to be tuned for NiMH instead of Li Ion). These two have some tradeoffs versus each other:
ICR has relatively more power density than IMR. That is, both are 4.2V max, but the ICR will be rated at higher mAh. That means that under many conditions -- but not all conditions, see below -- the ICR will probably last longer.
IMR can support higher discharge rates than ICR. There are some lights -- witness the 1x16340 lights that are putting out 500+ lumens -- that pull amps that are at the edge, or over the edge, of what an ICR can support, whereas an IMR won't even be breathing hard. Also, it is common that when you stress an ICR this much, voltage sags and it runs less efficiency, so that in extremely high-drain demanding applications, it sometimes happens that an IMR can last longer than an ICR, even though the ICR has a higher mAh rating.
ICR is less safe than IMR. That is, ICRs can go into thermal runaway easier, and that can mean an extremely unpleasant reaction ... in the extreme, your flashlight blowing up or house going on fire. Much more difficult to get an IMR to do that. As a result, ICR can be bought with or without a protection circuit. Depending on your state of knowledge -- right now, based on your questions, I don't think you should consider buying any Li Ion unless you learn a bit more -- you might stick with a protection circuit for now, if you go ICR. IMRs do not have protection circuits, because their chemistry is safer; you might ruin the battery if you mishandle it, but you probably won't blow it up.
My general guidelines:
- Understand proper care and handling of Li Ion
- Buy quality cells and a quality charger. If you're tempted to cheap out, don't play with Li Ions at all
- I stick with protected LiCo anywhere that works well. In a light that has circuitry that prevents overdischarge, and it can benefit from IMR's greater discharge rates, I go IMR. If the light can't handle a 4.2V ICR/IMR well, but can handle a 3.6V IFR, then I go with IFR. I don't do unprotected LiCo. Those are just my personal guidelines, there are smarter people than me with different ones.
