I wouldn't even say Mahrez has been that great for Man City. He's been good for them, yes, and perhaps did better than many other players who left us but
a) he's never been on the "key player" level at Man City in the same way he was at Leicester City (of course, that's difficult in a star-studded squad with rotation and so on)
b) he was often brought on / subbed
c) he never topped his total PL goal tally of 2015/16 (17) and 2017/18 (12) there.
Of course, he's done better than Maguire, Drinkwater etc., and he's won many titles to which he made important contributions, so I'd say he's done well but not great.
Edited to say I wonder what he will later look back on as his personal best season ever. 2015/16 with us, him being a stand-out performer and immortal? Or one of the multiple trophy-winning seasons at Man City where he's rather been "one of many"? Not so easy to answer perhaps!