I usually avoid these kinds of threads, as I'm soo indecisive.
But, 20 is a fair enough number, I don't have to narrow it down
too much.
And as you can see, I still had to cheat a bit 
I basically made my list with the thought of 'If I had to live on an island and could only bring 20 CDs...'
In which case I'd of course bring 20 CDs of mp3s 
Anyways, here's my list, I think:
In no particular order
The Cure - Pornography
Joy Division - Heart & Soul (box set)
Nine Inch Nails - Downward Spiral/The Fragile
TooL - Ænima
Moby - Play/Everything is Wrong
Beastie Boys - Check Your Head
Paul Van Dyk - The Politics of Dancing
Skinny Puppy - Bites and Remission
Ministry - The Mind Is a Terrible Thing to Taste/The Land of Rape and Honey
Fear Factory - Demanufacture/Obsolete
Smashing Pumpkins - Siamese Dream/Gish
Deftones - White Pony/Adrenaline
Underworld - Dubnobasswithmyheadman
Paul Oakenfold - Tranceport
Tricky - Maxinquaye
Portishead - Dummy
New Order - Substance
DJ Tiësto - can't pick just one, sorry.
Jane's Addiction - 3 for One Box Set (first 3 albums)
Depeche Mode - The Singles 86 > 98
Aphex Twin - Selected Ambient Works, Vol. 2