For upmarket, I really suggest Jogoya Japanese Buffet. The buffet is insane, and youre looking at about maybe 25 AUSD, which is 19 bucks USD>
Its in the middle of the Kuala Lumpur CBD, so you know its pretty high quality, and very expensive for the locals. (Lunch can be had for 70 cent USD.)... But for foreigners its pretty cheap for an epic all-you-can eat.
Its a Japanese Buffet, but also so much more. Korean food, chinese food, indian food, western food, seafood, hotpot, dumplings, dimsum, sushi... you name it they got it. Its humungous. Definitely worth a visit if youre ever in KL for a holiday, or a plane stopover.
Its setup like a market, with different stations/stalls for each different cuisine.

For the local stuff, cant go past Banana Leaf Curry Rice. Its like a curry rice which is served on a banana leaf. The best place Ive been is in a place called Bangsar in Kuala Lumpur, which is 15 mins from the CBD. On the street called "Jalan Bangsar 1", there is a great local place that does it really well. Great experience to have. Theres quite alot of these different places around KL, and they'll mostly be of exceptional quality.