Doz says:
... but I've never actually played an 800
than you probably should before you decide which one to buy, as it would probably suit your type of music better than the other two.:)
After all, there's no coincidence that most punk, hardcore punk and post hardcore bands that used/use Marshalls play JCM800 as it can handle well anything from hard rock to disonant sounds of alter rock.
But if I'd have to compare them I'd say that the JCM 800 is the most traditional sounding, has richer tone and less gain than the other two,so you couldn't exactly play thrash/death metal/grindcore/ without any aditional distortion pedals.
JCM 900 has observably more gain than JCM 800 and is a bit darker more modern(think 90s rock/metal sound) sounding than jcm 800.
JCM2000 is, as far as I'm concerned, just a big, more verstile, brother of JCM 900, with even more gain, more features than 900.
This is of course a very generalised opinion, cause there are differences between the models inside the particular series(for instance…reisued jcm800 sounds so different from the ones made in 80s that it hurts,same goes for one/dual chanel versions etc) and it's hard to give an objective picture.