For single battery mech, the CSMNT with Shawty cap. The downsides are 1/ these can be hard to find now, and 2/ to get the best flavor from them the coils need to be positioned at a close distance from the air holes and at the same height as the air holes so as a result the edges of the post block are making it impossible to use coils bigger than 2.5mm ID. But the flavor and feel of the vape are right up there if building it in this fashion (with 4.5 wraps 2.5mm ID Ni80 aliens 27/36 dual coil at .11 ohms and using a single Samsung 20S battery, which I can't factually recommend to anyone due to potential unknown factors that might affect the safety especially if you don't know what you're doing, but then again if you don't knowwhat you're doing you probably shouldn't be using a mech as your primary choice anyway in the first place, and, unlike some self-defined "experienced" vapers I don't try to push my own personal opinion on others, instead I just make the common observation that on a single battery mech almost nobody vapes at anything higher than .11 ohms).
For series mech it is a very close call between the various options from Purge Mods, the Mason V2, the Deathtrap, and the Unholy V2. It almost strictly depends on what coil build I want to put inside, but the Mason V2 will probably be very hard to find now. Please note, I pretty much striclty use mechs these days. They give me better flavor and especially better feel of the vape when compared to a regulated mod. There exist several advanced coil types that, while they aren't necessarily THE most advanced ones of them all, produce significantly better flavor (and also feel of the vape) when compared to, say, any pair of fused clapton coils. For example, in my book a pair of the alien coils I described above can easily give 20% better flavor than fused claptons can. So if you don't already know the basics of advanced coil building and you don't want to learn, none of the stuff I told you is going to matter that much. (Not trying to point any fingers here, but IMO some certain individuals on the internet appear to be stuck in some kind of fantasy universe when it comes to talk about advanced coil builds and/or mech mods, and about what exactly defines "high end" or what doesn't, even though this hobby is always part science, part subjectiveness of course.)