Isn't this kind of like asking what the best religion is? E Cigs and Mods are chosen around personal preferences and personal experiences. Drop into the Chuck forum, and their users will swear by it. Same thing in GG, etc.
Safest: Well, 3.x Volts is "safer" than 5V+, but with good protected batteries and care, all of the professional mods can be used safely.
About the only thing that jumps out (to me) as a differentiator is whether is has an on/off switch in addition to the actuator.
Most Reliable: The metal mods built with a solid switch are more durable, but more reliable? If a mod is breaking all of the time, it doesn't last long (or have many fans). If you are not wanting to worry about being careful while you vape, lower voltages have less tendency to burn out atomizers, but that combats vapor production in many cases.
Most functional: Other than vaping, what functions are you looking for? Adapters are great, but they can be used on anything, so that's not a determining factor. One nice to have is a USB passthrough mode that charges while you vape, but it's not neccessary
Most vapor producing: Now the war starts. Higher voltage usually means more vapor, but combats reliability and safety. There's a thread for every type of mod claiming that it has way more vapor than anything else on the market.
There are dozens of mods that meet the qualifications of safe, reliable, functional and great vapor production. You need to go deeper to find differentiation.
To pick a mod, you need to pick your configuration.
Some questions you might want to add to your list:
What form factor do I like (box, tube, smaller, larger, etc.)?
How portable does it need to be?
How does it look (to me)?
How is the support? Read a few user threads to see if there are complaints about responsiveness.
How much / how easy to get are the consumable parts (atomizers, batteries, carts)?
Do I plan to Drip, Dip or use carts / cartomizers?
How much battery life do I need?
Which of my initial four qualifiers is the most important to me?
After you answer these questions, you can narrow your choices down to less than half a dozen. At that point in time, you are down to how much you want to spend, and who can get it into your hands the fastest.
Good luck with your search.