What's the easiest way to get the highest quality still from a DVD on a mac?

Download VLC and watch the movie through that.

I haven't tried this, but i assume it works! Press apple key, shift and 3 at the same time, that will give you a screen grab.

Actually, sorry, it doesn't work, i just tried it.

If you don't mind using terminal you can do it through there. I don't remember the exact command but if you google it it should come up.

apple quicktime pro $29 but allows single frame export as jpeg. you just cue it up them file/export and use drop down settings... Use it all the time plus a ton pf pther features-it's really the swiss army knige of apps on mac (and now PC too.) plus H.264 codec rocks!