Any ecig is designed to turn liquid into vapour. Just about every design of atomiser uses a reservoir of liquid from which a wick draws up a certain amount of liquid which it puts in contact with a coil. The coil gets heated and vapour is produced.
Everyone has different tastes and that includes how strong you want your vape to taste, how much throat hit you'd like, how much nicotine buzz you'd like and how much vapour you'd like to produce and also how the vape 'feels'
For the vast majority of people an eGo or a vamo or a n other regulated mod with an evod or a Protank or a vivinova or a cartomiser will provide a decent vape.
Aside from the advantages listed in the first couple of replies in the thread a well setup mech mod with a suitably coiled atomiser on it will give a 'better vape' than a regulated mod will.
A Mech mod is literally a battery a connection to screw in an atomiser, a battery holding thing (usually a tube) and a switch. The higher quality ,mechs have higher quality workmanship, higher quality of materials, higher quality of quality control and branding.
A mech will give a 'smooth' feeling vape. One of the Chinese regulated devices will give you a 'pulsed' vape. In a regulated mod it's using Pulse width modulation somewhere in it which is where it fires the atomiser at 6v for some fraction of the time the button is pressed, and how long is part of the 'off cycle' and how long is part of the 'on cycle' depends on what you've set it to.
A ProVari uses PWM but does it better than a Vamo, so a ProVari vape feels smooth despite the fact that it's also pulsing.
In order to get the best out of a mech mod you need to know what it is that you want. More/less vapour? more/less flavour? more/less throat hit etc etc and you also need to know which of the variables you need to tweak to get there.
If you know how to improve this on a mech you can do the same on a regulated mod almost as well. You have a easier time of things on a mech mod as the mod doesn't have limitations on current imposed by any circuitry/wiring in it. At the same time it doesn't have any safety features in it so you need to know what you're doing and why.
My preferred vape is a Kayfun on a v3 ZMax. I have a couple of mechs but I prefer the vape I get from the v3 and I like being able to tweak it a little at different times of the day (more power in the mornings, little less in the evenings) and it turns out I quite like the 'rattlesnake' sound you get on a Zmax.