The basic. The one that is easiest to just pick up and play. You can take it anywhere, no amp, effects or power needed. Simplicity itself. The archetype of this archetype is arguably a dreadnought such as the Martin D28. The grandaddy of the dreadnoughts. Evolved in pre-World War II America as a folk and bluegrass instrument to be flatpicked in a band with fiddle, banjo and accordion. Solid mahogany back and sides, spruce soundboard. Paul McCartney wrote and recorded Blackbird on a D28.
Another classic body shape is the Martin OM— quieter with less bass and more suitable for fingerstyle. So at stretch one could push that there are actually two flat-top guitarchetypes essential for any collection.
Although Martin arguably remains the big daddy in flat-top acoustics, Gibsons also have iconic status. Think Bob Dylan. In recent decades Taylor have emerged as the third big name in premium flat-tops.
You can now spend as little as $100 on a new buzz box from Guitar Center or absolutely as much as you want on a custom-built Santa Cruz (beloved of one Declan Macmanus), McPherson (with their innovative offset soundholes and cantilevered necks), Collings, Bourgeois among others
I have also always had a hankering– after falling in love with, but not buying, a vintage Hofner President in about 1988– for an archtop acoustic or ‘jazz box’. They just look so cool. If I could get one now for a decent price I would. But I don’t think an archtop qualifies as a ‘guitarchetype’ on its own. Just some kind of subdivision of the overall acoustic lineage, where the essential guitar would have to be a flat top. I am sure your opinion may differ. A similar logic should probably be applied to the resonator or Dobro guitar, the classic acoustic slide blues instrument. You could also make a case for a nylon string classical to be included. Fair enough, but first show me one classic rock’n’roll track that includes it and even then I’d argue it’s a sub-division of the flat-top. Unless you’re serious about playing classical, you are always going to get more use from a steel string guitar.
So there you go, you have to have an acoustic. Probably a dreadnought or smaller more subtle fingerstyle or ‘folk’ flat top. I have both; a cheap Sigma dread (in storage back home in Helsinki and in terrible shape last time I played it) and a more expensive Breedlove fingerstyle type (bought recently here in Arizona).