I'm not sure I can say which album (CD) I like the best. My three favorites are: ELPS' first (titled Emerson Lake and Palmer), Tarkus and Trilogy.
If, today, I had to say which one's best, I'd probably lean more towards Trilogy.
Tarkus is also very good with lots of synth parts.
#1 is more Hammond and piano but does include "Tank" which, in my opinion, has one of the best lead synth lines ever written, and also includes "Lucky Man".
If you don't mind live recordings, Pictures At An Exhibition is yet another good choice.
Brain Salad Surgery (again, in my opinion) falls a little short of the ones mentioned earlier.
Works Volume 1 and 2 are much different than all of the rest but are still worth the purchase. Volume 1 is a double CD with disk 1 split between compositions of Keith Emerson and compositions of Greg Lake (and Peter Sinfield; ala King Crimson) and disk two is split between compositions of Carl Palmer and the three (ELP) together. This is also where Emerson began using the Yamaha GX-1 featured on "Fanfare For The Common Man".
I haven't heard any recordings made after Works V2 so I can offer no comment. However, if you appreciate pure talent and love synths, especially Moog, any of the above will be a real treat.