Most reliable mod for under $100 that'll last forever??

[Z-Lee]

There are regulated mods out there that have IP67 rated PCBs. Although this isn't going to completely prevent all types of failure (overheating, extensive use, abuse, etc), it can help knock out some of the potential causes over longer terms such as moisture and corrosion, esp. if you plan on storing them.

I don't remember all of the mods that I've seen this on, but the one that comes to mind is the Gaur 21. I believe the Forz TX80 as well. I'm not sure if most or all mods that are IP67 rated can claim that rating for their PCBs as well, but it can most definitely not hurt.

 

[UncLeJunkLe]

I'm not a fan of mechs but I'd sleep better at night knowing if everything else failed and it was between a pod or a mech I'll take the mech everytime.

Take a look at this one.
https://www.fasttech.com/p/9450104
It's a squonker, but you can use it as a regular mod if you're not into squonking (or you can go down yet another rabbit hole and start shopping around for squonking RDAs lol). Along with some older tube mechs, I bought a couple of these squeezers as vapocalypse insurance.