I'll admit, I created this poll thread because I'm biased - this album is not only my favourite of all the Motown quads, it's also my favourite of all of the Temptations collaborations with producer Norman Whitfield.
Obviously, it doesn't have a breakout hit like All Directions' Papa was a Rollin' Stone, but for me this is a much more consistent album, and has a great variety of styles, from ballads like I Need You and Heavenly to uptempo numbers like Let Your Hair Down and You've Got my Soul on Fire. But like a lot of music of this era, it's the "message" tracks that are the standouts for me. Ain't No Justice, which talks about the idea that there aren't really equal rights until black people aren't viewed with suspicion by default (something that still seems very timely today) and 1990 juxtaposes the difference in fortunes between the space race and ghetto poverty. And of course, a Whitfield/Temptations album wouldn't be complete without an extended track, which you get in the form of the slow-burning Zoom, the nearly 14-minute long lament for the problems of the time (higher taxes, drug use, etc.). The whole album's dark, disenchanted, psychedelic vibe seems perfect to me for Watergate-era America.
Apparently the Temptations often chafed at working with Whitfield, feeling like they were often supporting characters on their own albums to Whitfield's production and arrangements, and on some of the previous albums I think they definitely have a case for that, especially the one that immediately preceded this one, Masterpiece. But on 1990 I think the two sides find an almost perfect balance - I love the production and arrangements on this LP, but the vocals get, at the very least, equal exposure, and the majority of the tracks are very much features for the various lead vocalists. It's a real shame that the two parties went their separate ways after this, because Whitfield produced some fabulous stuff for the Undisputed Truth, and the Temptations basically fell off a cliff in both quality and commercial popularity shortly after this.
Like I said, I love this album, and the quad mix (even off a 40+ year old LP) is excellent - and at times very different to its stereo counterpart. Take Let Your Hair Down for example - in the stereo mix, the majority of the backing musicians drop out at the beginning of the track for an a capella vocal, whereas the quad mix has a full-band arrangement throughout. There are lots of differences like this throughout the album, but it somehow always manages to maintain, and often enhance the vibe of the original stereo mix, which I should say I stopped listening to once I got the quad version, as it's that good.