Just go there, full stop! I've lived in Khon Kaen since I was born and have tired Gai yang all over Thailand, but I always come back to Rabeab for its famous "Khao Suan Kwang" Gai Yang. Khao Saun Kwang is a district in Khon Kaen and is well-known its secret recipe of Gai Yang. If you look for traditional Isarn food, look no further. Gai Yang here is by far superior to SF Gai Yang (which is another place in Khon Kaen selling Gai Yang).
Rabeab restaurant is my most favourite restaurant for Gai Yang, Laab, Tom Seab, and Som Tum - these four main dishes are consistently good. Many people go there just for its famous Gai Yang - which costs around 120 - 180 baht per whole chicken, depending on the size. Make sure you order sticky rice to have with Gai Yang. Don't be put of by the lack of English menu, you can just go to the front stall and point to the things you like before heading to a table.
The place is not air-conditioned but there are plenty of fans to keep you cool. Also, you won't have to worry about the hygiene of the place because both the seating and cooking areas are well kept and cleaned. There are parking spaces in front of the restaurant. The place could be difficult to find and may be difficult (although not impossible) to get there by foot from the city centre. Just get on a Tuk Tuk or a taxi, they should all know where it is.
Seriously, I cannot recommend this place enough. This is a HIDDEN GEM little known among tourists. As a local with food insight, this Gai Yang place should be ranked in the top 5 of must-eat places in Khon Kaen. Don't think twice, just go. And don't forget to order the "Tub Tim Grob" after your meal - this is a Thai dessert, translating to crunch pomegranate; it is not real pomegranate but it looks like pomegranate seeds (made from water chest night and red food colouring), served in sweeten, aromatic cold coconut milk!!!! DELICIOUS!
NOTE, however, they don't open in the evening! It only opens for lunch and runs out of chicken quite quickly towards mid-afternoon, so make sure you go there early around 13.00.
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