General

Haven't played for a few months and I'm not too sure about their 5th Job skillsets, but as of now:

-Dual Blade has very high burst damage and is very mobile. Sadly he has the WORST skill range in the game so it's up close or nothing, which can really hurt DPM if you have to dodge a lot.
-Phantom has good damage output and is super duper flexible as a class goes. Levels very quickly with HS and your favorite mobbing skills. Personally, I find him to be really boring though.
-Mercedes is the hardest class of these to play. She has some very good DPM if you play her correctly, if you don't play her correctly then your damage output will be VERY inconsistent. She can be really difficult to master as a class. Kind of like the Zero of bowmen.
-Kinesis... I don't know. Used to be really strong but haven't played since release. Was a pretty interesting class to play with some wicked burst damage I'd say was on par with Dual Blade.
-Luminous isn't that great of a bosser as far as damage goes. He's good in other areas such as self heals, a good teleport, and some great range for his skills. He can be pretty flexible but his DPM is pretty blah without Equilibrium activated. Also worth noting he's easily the fastest leveler in the whole game.

But this is all on Reboot as you said, it doesn't really matter since real money isn't on the line. A little saying me and a bunch of people I used to play in Reboot with was "The strongest class is the one you don't get bored of." Basically it's the one you find the most fun because you can't transfer equipment over so you're not having to restart character after character because you get bored. Buffs and nerfs come and go so there's no telling how long that class will be top-dog or bottom of the barrel. I know it's not what you wanna hear, but pick what you find to be the most fun.

Thanks for the advice guys and girls ^w^.I'll probably go with Phantom then. As for playing what I enjoy, i honestly enjoy all of them very equally as i have gotten all of them to lvl 100+ at one point and tried them out.

I've mained both phantom and db, and db was definitely the better bosser of the two. But phantom was much easier to level, which is something you might want to take into consideration because leveling will be very important when fifth job is released. Luminous is a mediocre bosser, although he did receive the equillibrium (buff?) in the most recent update. Kinesis sucks against any boss that moves, and I honestly know nothing about Mercedes (other than the fact that their fifth job is basically a shadow partner).

Phanto op asf

Probably Dual Blade or Phantom but take my advice please: Play what you enjoy. Being the "best" (which is completely subjective) won't mean much if you're not having any fun.

Depends on how you control them.
If you like their play styles then play them all.