I am going to put drywall on my basement ceiling. Is it best/required (due to building code) to use a flame retardent 5/8" board?

You do not need 5/8 fire rated board for a basement ceiling. Think of it like this, there is not ceiling at all there right now so fire retarding isn't an issue.

That is what I thought. 5/8th inch is a good choice however correct?

I would use 1/2" if you have 16" centers to screw two. I don't know your code there but that's what's common because of floor joist centers

Just glue and screw

You may need it for the mechanical area for code. Code have is 5/8 on ceiling and walls when we close in the mechanical room. There is also the issue of Air for the heat plant/water heater. As strange as it sounds louvered bi folds pass code for the door end of it.

I agree 5/8" is just overkill and too heavy for my old bones. 1/2" should be fine but I'd even think about faceless insulation for sound deadening.

Hi I own a drywall co in green bay wi the rule to follow with drywall is you need 5/8 sheetrock for 24 inch centers trusses the reason is 5/8 rock will not bow down or pillow down if you have 16 inch centers 1/2 will be fine in new homes we use 1/2 in on basement ceilings all the time the only code that I am aware of in a house is you need 5/8 in rock in your garage for fire code I always tape the house walls and ceiling that way the inspector always passes it