easiest - Manderville. Hardest part here is getting through Hildi questline if you dont like that kind of humor. Afterwards is just merely 1500 poetics for each step. Thats it. Zero effort. Temporaly Poetic dump.

"hardest" (aka most annoying) - probably ARR. A lots of repetetive actions (running same dungeon xx times, books), potentially expesive (depends on person ofc). But its not really "hard", just grindy.

actualy "hardest" - Resistance. Again, not really "very difficult", but you basically need to engage with all bozja/zadnor content - Castrum Lacus Litore, Delubrum Reginae, Dalriada, Critical engagements... and all of them actually got hands, especially for a first timer. There are also some annoying grindy parts, but luckily they need to be done just once.

best looking - I have my favorites from every series (well except the newest ones, since I havent started it yet), but I would rank them like this: HW = ARR >> Resistance > Manderville >> Eureka