Easiest? I found Cadena to be moderately easy. Keeping your skills up to maintain your damage buff is the hardest part, but even then it's not that bad. I dislike Mercedes dash skills. I find their damage output not worth the hassle in low levels. Zero I just flat out don't enjoy. Being bound to a questline for however many levels and then theres way too many skills to use to be effective. I've never played blaster. They're on my list to try though. I heard they're fairly demanding. Mihile shouldn't be that tough. I haven't played one in years though.
In my opinion, Mihile is the "easiest" (least mechanically intensive) out of that list. As long as you know how to proc Royal Guard, the rest is business as usual (i.e. just spam a few skills). Blaster, Zero, Cadena, and Mercedes are more combo-oriented, requiring more apm to chain skills together for buffs or to attain optimal dps. I listed them in what I believe to be descending order of difficulty. Blaster combos are extremely mechanically intensive and many players rely on macros to execute their optimal rotation because it requires so many inputs in a small window of frames (mobbing and bossing). Zero and Cadena require you to link skills together for optimal dps (mobbing and bossing), which may sometimes involve displacing your character to less than ideal positioning. Mercedes also requires you to link skills together for the Ignis Roar buff, but in reality it's just linking stuff like Unicorn Spike, Spikes Royale, and Wrath of Enlil every now and then while holding down Ishtar's Ring if you're bossing. Mobbing as Mercedes can be more complicated since you might use your fancy floating combo to train. As for which I'd rather play as, my decision is based on enjoyment rather than complexity. I've always wanted to fund a Mercedes, not for the combos but because I'm a sucker for hurricane-like skills. Also unicorns.
I found Blaster combos really intimidating at first, but once I played the class enough, the high APM gameplay felt so natural it eventually stopped becoming an issue for me. I admit it was a pain in the ass (and wrists) to learn, but once I overcame that hellish learning curve all the pain and tiredness went away. Now I can grind for 4 or however many hours straight and my hands would feel the same as if I were playing any other class. That aside, Mihile would be my top pick if my ping allowed for it. But I live in Australia, so... Blaster is my choice. This class gives me so much freedom over how I use my character thanks to the animation cancels, movement options, and mobility which I have precise control over (example). Being able to tank super knockbacks+OHKOs is also a gigantic plus. In a way, I find Blaster easier to boss with simply because all those things give me an advantage that many other classes don't have. Which brings me to the subject of Zero. In contrast to Blaster, Zero is the hardest for me to play because I find their gameplay a little too restrictive for my tastes. The forced movement on their core damage skills as well as those ridiculous animation locks make bossing difficult for me. You have to plan your positioning and time their skills perfectly so you don't do stupid things like fly into 2 Magnus meteors. That playstyle just isn't for me. Other people manage to make it work, I just can't do it lol
Mechanically, Mihile pretty much has nothing insane. The only 'difficult' part of his kit is his parry which is so heavily affected by my Australian ping that I don't rely on it. As such, Mihile is pretty easy for me to play :^) Either way, I have a huge love for warriors and Mihile is great.
"...and many players rely on macros to execute their optimal rotation..." I don't use macros... holds up a broken wrist Also so many Australian users here owo