Honestly, Super Junior M has the best Chinese pronunciation I've heard from a mainly Korean group. Of course, they have Henry and Zhou Mi. EXO has Kris, Lay, Luhan, and Tao. But Kyuhyun has pretty perfect pronunciation, Ryeowook is almost there, and everyone else on SJ-M is pretty good with their pronunciation.
Whereas although Chen and Xiumin have improved since MAMA era, they still won't be able to compare to SJ-M.
Fun Fact: My dad can't understand it when EXO is singing in Chinese (the Korean members) but he can understand SJ-M.

Chen's Mandarin has been okay but I'm not sure if it's improving since they're not even promoting in China anymore.
And Xiumin is just not...He's never given the opportunity to.
SJ-M has had many years to improve/hold comebacks

I don't know Chinese so I can't tell but that's just my opinion.