xy are the easiest imo. could toss in oras too. i like the new exp share introduced in gen 6, but i think it wasn't implemented well. i think they got it right in gen 8, it's more balanced there imo, but not so much to xy which can lead to quick overlevelling. they're fun games regardless, but unless you're limiting yourself in some kind of capacity, they're a breeze.
edit: i do agree with dawn, too. generally if you know the type chart and what you're about to face, you'll be fine. there can be some wrenches thrown in, but usually grinding out more levels can solve that issue. and because of the implementation of the new exp share in gen 6 and onward, i think players tend to view them as easier (like i just mentioned) because you're having to grind less bc the whole team is getting experience altogether. which means you're spending less time to prepare for a gym or boss battle which, to us, results in thinking 'oh this is easier bc i didn't have to do that much here'. i know some people still turn the thing off and say they're easy though, but i think another thing about that is that a lot of the people that say this are older fans...and we have a good grasp of the game mechanics by now so we can squeak by with this. the gyms i thought were hard as a kid in rse are not hard as an adult, for example.