op.20 and op.33 are essential works (in fact these two sets are probably the most important ones historically whereas one could pick any set of the later ones). Fortunately these are not too hard to get separately and also in rather different approaches. For op.33 I would also mention the Weller (late 60s Vienna) and Casals (modern HIP influenced)
Choices get rather slim for separately available op.1,2,9 and 17. Again, make no mistake, op.9 and 17 are not really early pieces, only a few years before op.20 and contemporary with pieces like the "Mourning" symphony. I am not sure if all/most of the Buchberger was available as twofers. I have their op.9 and quite like it. Angeles (box only) is very good in the early pieces, I think. Kodaly is o.k. (I heard one disc of their op.17)
opp.1 and 2 are really early (probably before 1760) and have been put in a different volume in the more recent Haydn edition, separate from the "real" string quartets (seems a bit silly, but I am not a musicologist and they have of course reasons like the different form usually in five movements and that the bass was maybe a violone, not a cello or whatever). Not essential, but evertheless, they are charming works and sometimes more than merely charming (e.g. the wonderful "duet" adagio in op.1/3).