As far as firewood saws, everything in my sig line is used for firewood duty, though given the trees I routinely take, the 261 and the 562 have been the go-tos. The 262 is a wicked, pissed off little bastard, the 550 is new to me and I've only used it once, so though I like it a lot, I have little experience with it. The 346 is also nasty but I need to tweak it a bit. The 201 gets used a lot for odds and ends and dismantling the upper canopies. The 372 and 9010 come out when there's large crap to be tackled. I usually have an idea where and what I'll be cutting before I go, so I grab 2-4 that will be the most useful and run whatever feels right at the time.
If it was only one, It would either be the be the 562 or 262 and I'd lean towards the latter as it's a bit lighter and quicker handling while still having enough power to get a lot done. The 562 has a bit more torque though and is decidedly smoother. Otherwise I'd say a ported 036/034, ported 6100, ported Efco CS62 or ported 357xp and it might be my absolute favorite, as I love the character, feel and handling, being an oversized 346.