Altman's Best Flick? -- Long Goodbye, No doubt
Altman has made some amazing flims - one of the best directors of the 70's, by far. Of course, he's also made a lot of crap films as well (Dr. T and the Women, Pret A Porter, anyone?) Nashville has been said sum up America as a whole in the mid 70's mindset from a cultural perspective -- political, emotional, musical, etc. Altman as auteur -- the overlapping dialogue, the impression of improvised acting (though Altman says most of his films are tightly scripted) disjointed, crazy quilt sequences, multiple strands of plot and character that come together at the end -- plus a dash of conspiracy, as was the theme in Mid '70s films (All The President's Men, Three Days of the Condor, etc.)
The Long Goodbye, though is his best film overall, next to McCabe and Mrs. Miller, MASH and California Split. It gels and clicks, the dialogue pops, all the performances are incredible, Johnny Mercer score, everything about it is brilliant. It also stands up the frequent viewing, while Nashville is dated, plus a bit of a slog to get through.
California Split has just been released on DVD and has a commentary track with Elliot Gould, George Segal and Altman. Altman says that of all of his films, this was the most improvised. It also is quite incredible, the way Gould and Segal play off each other.
Oh, and Short Cuts is insanely overrated.