An HD-28 would be great if you were playing Bluegrass. I don't, however. But they make many fine guitars, and I would not even begin to say that any of them were "better" than another.
My favorite Martins, overall, are OOO's, OO's, and O's. I tend to favor mahogany back and sides, so I usually like 18's over 28's, and I much prefer old ones. Age makes a difference, and in guitars it is a good one. Probably my favorite Martin ever is a 1911 O-28 we had in the shop many years ago. Amazing guitar.
And the best sounding acoustic I have ever heard is a 1930's Gibson Jumbo (probably an Advanced Jumbo, though we are not really sure) a friend of mine has. The damn thing is like a cannon.
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