I wouldn't really consider Modern Snack Bar a diner but we do like it. It will re-open in the Spring.
Our favorite Suffolk diner is Lake Grove Diner on the South side of Rt 347 just East of the Smithhaven Mall. Open 24/7 as all diners should be. K.I.S.S. and you won't be disappointed.
Island Grill Diner E Patchogue just off Sunrise is excellent for breakfast lunch and dinner. Massive portions so you always have lunch or dinner leftovers. A great value.
Cutchogue Diner is good but a little pricy for diner food.
Riverhead Diner near the hospital is ok for breakfast.
Hampton Bays diner is good for breakfast but pricey.
For comfort food try Phil's in Wading River. Very good inexpensive restaurant/sports bar food in a nice setting. Great burgers and specials every night. Lots of tables away from the bar too.