For me both Great and Ultra Leagues are very welcomed because of variety. There's plenty of trashy mons I would have never given a damn in my collection that are now real powerhouses like low level Blastoise and Flygon. Ultra League is nearly every second weather boosted mon, but then there is Giratina and the brain puzzle of how to counter it. I did well with Ice HP / DG Togekiss until I faced an Ancient Power one that kicked my ass.
[Opinion] Toughest League
Opinions on what the most challenging league is?
My vote goes to Great League. It's such a world of difference using Pokemon with CP that low and apparently I don't have much at those ranks to put together a cohesive team. I've found myself getting punked on by Carnivine of all things, who is surprisingly really powerful at that league. Power Whip took out my Flygon that was somewhere between 70-80% health.
I'm not a tryhard by any means and I don't have a whole lot of battles under my belt to begin with but Great League is consistently the only one I recall getting spanked at.
Answers
How about good ol' Blissey with them Gleams or a Lick Snorlax? Alolan Muk? Tyranitar?
And, wait a minute, is that Wigglytuff doubly-resisting both Ghost and Dragon, and hitting the poor Giratina with Feint Attack and Play Rough? Somebody call the pokepolice! But wait, there's now Mawile joining in, triply-resisting Dragon and resisting Rock. Oh dear, oh dear.
(well, yes, Mawile lost the PvP fast move lottery, but then Ancient Power is not so poweful either)
I almost never lose on any of the levels...however, I’m forced to do 2 (if not 3) battles a day against myself. Even when I challenge myself it can take up to 30+ seconds for the request to get to the other account. Am I supposed to request everyone on my list and sit there a minute each time to see if they accept? Did get my 2nd Stone today—wouldn’t have happened w/o my other hand.