Best? Lot's of opinions here I am sure. Plywood? Perhaps he's steering you to having it nailed down, or thinking the plywood could bridge high and low areas of the existing subfloor for a floating floor?

My personal favorite because I'm very skilled at it, is a direct mastic glue down floor.

Glue down installation

Floor preparation on concrete - Get it flat first!

Almost all my wood floors over slab are direct mastic glue down as mentioned above. I prefer the sound (lack of) and solid feel.

How old is the slab? How flat is the slab? What's sitting on it right now? Let's start with the basics. Old slabs (built before 1990's) can have moisture issues. Without the right adhesives (and you would be surprised at how many adhesives that say they are 'unlimited' have rather sad limitations for moisture) you can get massive failures very very quickly.

I'm always surprised at the 'glue down' the plywood and then nail down the hardwood. Why not just glue down the hardwood? It requires the same adhesive...so why not go ahead and do the hardwood that way...and skip the plywood.

A floating hardwood floor (with proper moisture barrier underneath) is the one least likely to fail (due to moisture). A properly prepared slab (all the grinding/filling/leveling) is going to produce a lovely floating hardwood. And if you want a 'solid' feel underneath, go ahead and use a lovely 1/4" cork underlayment underneath the hardwood (but over top the moisture barrier). It is a really nice combination.

The slab is 6 months old and fairly level except for a couple of places with moisture barrier underneath. Nothing s on the slab now. Thanks!

So long as the moisture levels have been tested by your flooring professional (not the 'guy' the GC makes install the floor) and the barrier has been proven to be intact, you are welcome to glue down the hardwood. It is still HIGHLY recommended that you use the BEST adhesive on the market...just in case.

The slab still needs to be LEVELED. The flatness of a freshly poured concrete is rarely good enough to put down a rigid floor (most pours are ready for carpet and that's about it). The subfloor preparation still needs to be dealt with. And again, this needs to be done by the wood flooring PROFESSIONAL that the GC HIRES to deal with this part of the project. Please do NOT let the GC give this job to a 'guy who swings a hammer'. This type of stuff needs to be dealt with by the specialists. Just like plumbing and electrical are dealt with by professionals, so too a flooring preparation and wood floor install need to be dealt with by specialists.