Mistweaver
Most fun: MoP
I loved using chi torpedo to heal, it was so exciting! I loved Jab Jab Uplift but that didn't last very long. Punching to heal was fun. The mastery (although it was OP on release and quickly nerfed) that created healing orbs everywhere looked so visually pleasing when looking around a boss platform during a raid. Blanketing the raid with Renewing Mist and then chaining instant cast Uplifts felt amazing. Mistweavers were the first healer that didn't feel like a traditional healer. It was highly mobile and its main heals were fast-cast or instant casts. Casting Soothing Mist then following it up quickly (0.5s) with an Enveloping Mist (the numbers for Env were terrible but it just needed buffing and mana cost nerfing) felt so smooth. I loved being able to recharge my mana with Mana Tea which again removed the boundary of having to maintain and watch mana like other healers.
Least fun: Legion
Blizzard really butchered Mistweaver's identity and brought it in line with all the other healers. Cast times were added to literally every heal except Renewing Mist, Chi Torpedo lost its healing and damage, and fistweaving was completely wiped out. I imagine they did this in an attempt to broaden the appeal of the spec to more players. However our numbers were terrible and Essence Font was a boring concept that became the staple part of our healing setup. No one was going to switch to Mistweaver if the numbers were bad, which they were. We drained mana which seems to have been an issue ever since they removed Mana Tea. They still struggle with adjusting our heal costs accordingly. They also ruined Soothing Mist by tacking it on the end of every heal. It healed absolutely nothing and provided no use. Overall the spec was just in an abysmal state.
Windwalker
Most fun: Legion
I LOVED the artifact ability Strike of the Windlord. The AOE ability that cost 2 chi and hit like a truck felt so fucking good to use. Transcendence also healed on use (Transfer) and it had a 25 second CD. Mobility was at its peak which was needed to stop DH from stepping on WW's toes.
Least fun: BFA
Artifact ability gone, say no more. Its replacement was Fist of the White Tiger yet all the two abilities share is the animation. FotWT hits like a wet noodle and is a chi builder not a spender to deal damage. It's shit. WW mobility was also hit fairly hard with a removal of the trait that made Transcendence a 25s CD, so now it's up to 45s. In addition, for absolutely no fucking reason they nerfed Flying Serpent Kick to travel for 1.5s down from 2.0s. I have no idea why they did that. Windwalker is in a position in BFA right now where they do nothing, bring nothing, and aren't needed. The mobility is outshined (DHs mobility reigns supreme), the damage is outshined (worst singletarget damage in the game right now), and DHs and Rogues are basically the same as WW but better. They just bring more and it's depressing.
For an in-depth analysis of the state of Monks, Preach did a fantastic video during Legion and it still holds up very well.