The great thing about Truffaut is that he spent virtually his entire career working for the same production company - with the result that a huge chunk of his output is not only available on DVD, but also that the quality threshold is gratifyingly high.
I can certainly recommend <I>The 400 Blows, Shoot the Pianist, Jules et Jim, Stolen Kisses</I> and <I>Day for Night</I> pretty much unreservedly - and I feel I should add <I>La Peau Douce</I> to that list, but it's ages since I saw it.
It's easy to find out what's available on DVD - just visit Amazon's <A HREF="http://www.amazon.com">American</A>, <A HREF="http://www.amazon.co.uk">British</A> and <A HREF="http://www.amazon.fr">French</A> branches and do a search on "Truffaut".
In general, the MK2 French editions are the best (superb transfers and exhaustive labour-of-love extras), but they don't always have English subtitles (some have subtitles on everything, some on just the main feature, others not even that - and I think <I>Fahrenheit 451</I> has non-removable French subtitles - it was Truffaut's only English-language film).
As far as I can see, the Tartan UK editions are sourced from the MK2 discs, so can probably be recommended (though I haven't seen any myself), and in the US it's a toss-up between Criterion and Fox Lorber/WinStar. The Criterion Truffauts have yet to be released, but it's probably a very safe bet they'll be superior to Fox Lorber/WinStar, whose track record is less than stellar. Don't bother paying through the nose for the out of print Criterion <I>400 Blows</I>, as its reissue in a superior transfer is imminent.
So if you speak French, it's MK2 all the way, if not, go for a mixture of Tartan and Criterion.