Reliable is sort of a pot luck game. You can get the one everyone says is reliable then yours breaks one day after warranty.
What can be said, is that
likely the most robust ecigs are the ones that don't use propriatry batteries and also don't fall apart. That would be like box mods, reos, provaris and other high end mods. You only have to replace a cheap replaceable battery in what is meant to be a unit with better build quality.
But if we're talking standard joe, the "workhorses" of the ecig world have to be the ego and riva. There's only a slight difference between them:
The ego is a regulated 3.2v device (it will be 3.2v until the last vape).
The riva is unregulated (starts near 4.2v? and ends up around 3.2v at the end or something like that).
The most robust cig sized ecigs are probably the "newest" kr808D-1 models branded as volt/bloog/halo g6.
Next is the heating element - some will last a few days, some will last a few months.
Cartomisers will tend to last a week each, cheap, disposeable and tend to work pretty alright in their lifespan. There are exceptions like clearomisers which can last months but can be a little fiddly.
Atomisers are probably the most robust heating element. They're sturdier, thicker
coils and cleanable. Expected lifespan is a few months. Worst downside is it's not very portable as it can't hold much liquid in it to take out with you.
Most people direct drip at home with these.
tank based heating elements like the ego T or ego C can last a long time, similar to an atomiser but they can be a lot more fiddly than any other heating element. As they sound, they're a more lasting solution to the cartomiser, holds more liquid and should last a few months (should).
There's a lot of other heating elements out there but those are the main categories.
For batteries, the ego or riva will probably get recommended the most and I'm not so sure about heating elements for those as it comes down to personal preference.