Hardest gen in your opinion

[LilyGardy]

To Grass Monotype Gen 7. Some of the Totem Pokemon are tough.

I'm yet to Grass Monotype Gen 9 but I found Gen 6 to be the easiest. Gen 1 was straightforward too as I started with those games and Venusaur and Vileplume did most the work.

 

[Diamond 97]

US/UM is pretty hard, it gives you a lot of options to make the game absolutely free/more difficult which is neat.

Gen 2 games are probably next hardest by virtue of poor levelling curves. There are some very tough fights and some extremely free fights more because of poor design than anything else.