Most challenging and wanted DPS class?

[Lastelli]

If you want to perform close to the highest possibile level, SAM is probably the most challenging melee. It has an extremely high skill ceiling, but a very low skill floor.
BLM has a very simple rotation, but requires a lot of effort to execute this rotation in relevant content because of its limited mobility, so it is generally viewed as a very complex job to master as well.
Both are very selfish, with little to no utility for the rest of the party, and have a very high personal dps and are generally welcome in most parties.

Then we have MCH, with a moderately high skill floor and a very high skill ceiling, but right now it's in a pretty bad spot since it's outclassed by brd in every possible way and scenario while also being harder to play at a high level. But if you're looking for a challenging job, MCH is a good pick and it's very fun to play.

[Idolon]

However, unless 4.05 really rectified a lot of MCH's Stormblood redesign, they are still not dealing as much damage as an equally-skilled Bard. Again, that's if 4.05 didn't fix the glaring issues with the job.

4.05 buffed MCH damage slightly, but made the glaring issues with the job even worse. According to data on FF Logs, MCH and BRD are fairly evenly matched up to the 75th percentile, but the most skilled BRDs are able to do significantly more damage than the most skilled MCHs.

Then we have MCH, with a moderately high skill floor and a very high skill ceiling, but right now it's in a pretty bad spot since it's outclassed by brd in every possible way and scenario while also being harder to play at a high level. But if you're looking for a challenging job, MCH is a good pick and it's very fun to play.

If OP is looking for a challenging job, it's not MCH. It used to have a moderately high skill floor in 4.0, but after 4.05 it has a fairly low skill floor and equally low skill ceiling. Avoiding overheating now requires little effort beyond remembering to use Cooldown once every 30-60 seconds, and in the event you do accidentally overheat, it takes so long to have done so that Barrel Stabilizer is almost certainly available to use again, making overheating not much of a penalty. The only things that are particularly challenging now are the things that shouldn't be challenging, like recognizing that overheat + Wildfire is rarely good even though all the tooltips lead you to believe they're intended to be used together, or actually forcing an overheat for the rare situations where it is useful.

[Selova]

snip.

This is all simply not true, out of all the dps MCH takes the cake in what is required to play the job optimally. The only reason people claim that BLM is challenging or hard is due to heavy movement fights, unlike MCH BLM's actual rotation is incredibly easy. MCH's entire kit is based around WF and OH and making sure you time your OH's with every WF and all your ogcd's and if you aren't OH with every WF you are intentionally gimping yourself. Contrary to popular belief, MCH is pretty strong, hence whey they are taken on almost every speed kill. The second most challenging job to play optimally is def samurai, but if you want to challenge yourself I'd say either SAM or MCH are a good pick.

[aeoncs]

If you want to perform close to the highest possibile level, SAM is probably the most challenging melee. It has an extremely high skill ceiling, but a very low skill floor.

I know I'm kinda necro'ing here but I'm wondering if you, and others, would stick by that opinion today. Right after SB was released many people were of a similar opinion but imo SAM is most definitely the easiest melee in Savage/Extreme content.
It doesn't get punished for "having to do mechanics", phase transitions or downtimes in the slightest if you use Hagakure correctly, which is probably the only moderately challenging part aside from Kenki/Kaiten management. It also has no buffs or cooldowns to sync with your raid, save for maybe holding on to Guren for 4-5 seconds for an upcoming TA. Tbh, it's probably about as easy as RDM in said content.

B2T:

Most challenging jobs to pick up and do well/ok with: I'd have to go with Dragoon and Summoner
Highest skill ceiling/most challenging job to maximize: Probably Bard, Ninja and Monk
Most wanted: Really depends on whether you're doing serious progression, speed runs or just casual raiding but Ninja > BRD = DRG are probably always first in each scenario.

That being said, I've barely spent any time with Bard & Ninja at 70 on an Alt, so some of that is guesswork.