This is pretty much impossible for me to answer. According to last.fm, these are my most listen-to albums:
1. The Knife – Deep Cuts (784)
2. All India Radio – Once A Day (688)
3. Dick Dale – King Of The Surf Guitar: The Best Of Dick Dale & His Del-Tones (648)
4. All India Radio – These Winter Dreams (642)
5. All India Radio – Permanent Evolutions (630)
The number at the end is how many times I have listened to a song off that specific album. Of course, the above just reflects the last 9 years of listening for me, and is further skewed by the fact that I have listened to a lot of A.I.R. in the last 4 years. It's an amusing list, but not representational.
I could only name four right now. These are not in any order:
Biosphere - Substrata: Probably the greatest ambient album of the 1990s; possibly the greatest ambient album, period.
Black Sabbath - Volume 4: The best album from the best period of the best metal band, full stop.
Tom Waits - Bone Machine: IMJO, the last great Tom Waits album. He took an extended hiatus after it. Everything that has followed has not impressed me nearly as much. Tom is still capable of writing (and performing) absolute gems, but none of his albums have been anywhere nearly as consistent as Bone Machine. I'd also list Rain Dogs, Swordfishtrombones and Frank's Wild Years, but I consider them to be one album in 3 parts (with Big Time being the live counterpart.)
Orbital - Orbital 2 AKA The Brown Album: The album that got me in to electronic music, over two decades ago. The US CD edition has a sticker on it with the review blurb "Pure, heavenly technology." That term was something of a rallying cry for Orbital fans in the 90s. Also, Seeing Orbital perform in 1999 remains the best live live electronic music show I have ever attended.