*There is no one "Best" modem for everybody. Unfortunately you actually have to try to find the modem which works perfectly for you on performance, strength and battery drain levels. The good thing is that from my lengthy 8 month experience with Galaxy S2, I'm happy to report that there are a few modems that seem to perform well geographically for most users. These modems are KE7, KH3, KI3, KI4, KL1, LPS, NEKF2, NELP2 & NELP3.
*Modems do not need to be matched up with firmware like it's done with ROMs and Kernels. For example, a KI4 modem can be used with a KL1 ROM and the other way round. The only thing you need to pay attention to is what model your SGS2 is. 9100 and 9100 modems are compatible across both models. However, 9100 model and US variants are not cross compatible since the devices are designed to work on different bands.
*Usually just flashing and then using the same modem for 3-5 days should tell you how well it performs in your are and with you Mobile Network operator. Please remember that it takes around 2-3 days for a Modem to "settle" and achieve optimum performance, so don't judge how good a Modem is straight after flashing. That's where a lot of people fail when asserting battery drain.