I do a lot of 1/72 gaming. The first thing I want to say is that the porportions are far more realistic and even if a given kit is technically excellent in terms of sculpting and reproduction of detail, it will never look good to someone used to
GW's style. In short, I don't think there is "specifically stuff that they think matches the quality of the best 28-32mm models" because the style is so different.
The smaller scale + different style means I can't think of a single example to show someone looking for an answer to that question. So I will say none of them. None match the quality of the best 28-32mm models. Now some metal sculpts might come close. Franznap is great, for example:
Overall, I think the boutique figure painter focus that 28/32mm has gotten over the last 15 years has pushed its quality well outside of what's largely available in 1/72. People putting out 28/32mm figures are pushing the envelope and really striving for people's attention for high end models. In 1/72, people are often thinking about dioramas and painting groups, or adding some infantry to go with some tanks in a display case.
There has been a great quality push in historical figures, but I think this has largely happened in 54mm and larger. For example, do an image search for 54mm Andrea painted.
To answer some other questions:
Do you play as single model on a base, or multiple models on a base?
I currently am playing ancients so I'm doing multiple models on a base. Usually like 8 figures on a 60mmx40mm rectangle. There are certain figures that might go on a individual base. I'm considering a rules set called Sword & Spear that has generals and leaders on small individual bases.
Anyone play "mass battle" scifi games at 20mm?
There aren't really many figures to choose from (can't think of any currently in production). Never heard of anyone doing it either.
I've done my usual searching online, but a lot of the pictures I find aren't of a full table in play.
It's primarily a scale for collectors, painters and diorama makers rather than wargamers. There are wargamers too though.
Places to check out:
http://bennosfiguresforum.com/ http://www.plasticsoldierreview.com/Index.aspx