The batteries that are best (and Mooch has said it himself), are the ones that suit your application. The new molicells out everyone is raving over are being touted as 'best all around' and to a point they are. If you had to have one battery that would suit 'most' applications, it offers high amperage and fairly decent mah. That said, the 'best' battery is for how you vape. What's best for someone on a mech mod won't necessarily be the 'best' for someone like me who vapes around 60-70w on a dual 18650 regulated and won't be the best battery for someone who vapes 15w on a single 18650 mod.
Assuming your mod is regulated and you're vaping 40w (the higher of the two wattages you mentioned), take your wattage, divide by # of batteries, divide by an estimated low voltage shutoff of 3.2v and divide again by 90% efficiency. Some power is lost from the battery to the coil so they're not 100% efficient. The 3.2v and 90% (or 0.9) are only rough estimates. Low voltage shutoff is where your mod says 'low battery'. It could be 3v, it could be 3.2 or even 3.4v.
So single 18650 mod, 40w / 1 battery = 40. 40 / 3.2v / 0.9 = 13.89 amps. That's the amperage that 18650 needs to be able to put out to prevent overheating it, stressing it beyond its capabilities etc. That would point you toward a 15-20a battery and there aren't any 15-20a lithium batteries with more than 3000mah. You're not going to find an 18650 that's 15-20a and 3500mah or 4000mah, not one that legit performs that way. Some good choices would be the sony vtc6, samsung 30q or lg hg2. In Mooch's 3 way test/comparison of those 3, the samsung 30q performed just slightly behind the vtc6. Usually they're a good amount cheaper than the sony's, so probably the best performer for the price for that application.
Dual 18650 mod, 40w / 2 batteries = 20. 20 / 3.2 / 0.9 = 6.94 amps. You could get away with using 8 amp 18650's with higher mah like mxjo 20a/3500mah (only a true 11amp battery), imren yellow or green 3400/3500mah batteries which say 25a or 30a batteries but only 10-12amp batteries. They're overrated for people who need 15-20amps of power, but if you stayed at 40w or lower on a dual 18650 mod you'd be fine. And you might get a little longer runtime out of them due to the higher mah.
If it seems too sketchy, there's nothing wrong with sticking with known good quality performers like the vtc6 or samsung 30q's. 400-500mah difference isn't going to matter a whole lot over 3000mah. Sure if you're comparing an internal battery pod or something with 400mah vs 900mah it's a big difference in how long of a charge you get. Above 3000mah it might mean 30-60min extra run time so not a ton of gain. And if you ever did vape above 40w on dual vtc6's or 30q's you wouldn't have to worry that you're right at the upper amperage limits for the battery.