Kinaara in Darnall does all you can eat Indian, Chinese and Thai. Admittedly the Chinese/Thai options weren’t too plentiful last time I went but still had a reasonable choice and was decent quality. Patoo on Ecclesall Road also does an all you can eat Thai buffet on Sundays which is really good…
Best All you can Eat chinese in town at the moment is easily Flaming Dragon up at Valley Centertainment. I do like Jumbo's but FD is ten times as good and the same price. If you've got a day rider, it's worth the extra few stops on the tram.