Reddit reviews Innovation in the Dynamics of Acid

Depends on what kind of music you like, he's done some pretty varied stuff over the years.

If you like ambiental stuff, definitely get Selected Ambient Works Vol. I & II. Volume II is a lot more abstract, hypnotic, and ethereal in terms of very loose song structures with little or no "beats", whereas Vol. I is a bit more of a straightforward outing. Both are worth getting definitely.

If you like his take on acid, look into the Analord series (12 EPs he released on vinyl only a few years ago). Some of those tracks are collected in "Chosen Lords", but there's a lot of other great tracks on those vinyls that didn't make it onto Chosen Lords.

For some old school type electronica, check out Classics; it's got some of his earlier works on it (including the pretty famous Digeridoo). The Richard D. James Album and I Care Because You Do are good examples of his mid-period kind of DnB mixed with other stuff. And the Analogue Bubblebath series is somewhere in between the last 2 albums I mentioned and some of the other more ambiental stuff he's done. Surfing On Sine Waves (released under the name "Polygon Window") also falls in the same category; pretty old by now, but a pretty good mix with some chilled out stuff.

As a side-note, if you like his acid stuff a lot, and you're also into minimalist techno (Detroit techno, glitch, that kind of stuff), check out http://www.amazon.com/Innovation-Dynamics-Acid-Universal-Indicator/dp/B00005LAMO/ref=sr_1_1?ie=UTF8&s=music&qid=1278996407&sr=1-1 . It's a continuous hour+ long mix of minimalist acid music made by Aphex Twin and some buddies of his and released on his record label. I really gotta be in the mood to listen to it, but if I am, it's great stuff.

Very vague and broad answer, I know, but I figured I'd point you in the right direction depending on what kind of stuff you're into, because there's really no good answer to what's "essential" IMO.

Enjoy. And while you're at it, check out some other Warp artists as well, like Autechre and Squarepusher if you haven't already.