Honestly... for mechs... depends on battery size. I prefer the power and weight over the 26650 batteries vs their 18650 and smaller counterparts. But if you want a good 18650 mech - hate to say it, but the gold plated V3 Flip. Why the V3? Flexibility, user serviceability, and customizability. Can fit various 18xxx battery sizes + Kick (should you need it), can be repaired by you - the end user - by purchasing the spare parts from V3Tronix, and can also be customized to your liking (using various color configurations, etc). Probably one of the prettier mods too. Only downside - its pricey - but if you got the cash for it, it's worth the weight in money. As far as 26650's are concerned - I'm unsure of them anymore. They seem to be dying off - people are moving away from the larger battery and most of the people in my area are moving away from mechs altogether. Everybody is "Regulated this... and regulated that...", but I keep my mechs around for a rainy day anyways.
As for resistance - there's a lot to go into about that. The battery itself will continuously TRY to push 4.2V (Fully charged) - 3.7V (average output) until the battery's overall voltage drops below 3.7V - at which point it will push that voltage to the coil. So wattage isn't something to be too concerned with when it comes to mechs. You need to focus more on AMP limitations / PEAK AMP LIMITS on your batteries.
.2 - .5 Ohms on ALMOST ALL BATTERIES WITH A 30AMP DISCHARGE RATE SHOULD BE SAFE - UNLESS THE BATTERY ITSELF OR THE CELLS INSIDE SAID BATTERY ARE DAMAGED / FAULTY.
So make sure you're buying reliable batteries / batteries with good reputations. Samsung and LG are putting out some storming batteries now.