I wouldn't know where to start with "best". There are so many great
places. As skiingfast said, all the casinos have a Sports Book that plays
the Sunday game - they can all be really fun. The upscale casinos will
have a different group of people than the lower scale ones so "best"
would change based on who you like to be around.
I tend to like the casinos downtown over the strip - a little rowdier means
more fun (to me). What hotel will you be staying at?