I have owned :-
BoR
Crunch Box
Menatone King of the Britains
Indyguitarist/Wampler Plexidrive
CMATMODS Brownie
Of these, the BoR was definately king of the lower gain, Jtm45-type tones.
The Brownie is a superb, uncannily close, emulation of an early-70's metal-panel Superlead.
The Crunch box didn't sound anything like the Jcm800 I owned. the CB had far less mids, was far more compressed and generally sounded much more like JCM900's that I have heard (never owned one though).
I have not yet heard a good JCM800 emulating pedal. The Lovepedal Purple Plexi 800 is the closest I've heard in clips so far.
The KotB always seemed to have a harshness to it in the very upper mids, that I could never dial out. I was actually very disappointed with it, considering how much it cost.
The Plexidrive was also a very disappointing pedal for me.
Other pedals I have owned which can produce some Marshall-type tones are :-
HT Dual
HT Dist
OCD
Jackhammer
Guvnor
These are all good pedals, but not at close to true Marshall tones as the earlier pedals I mentioned. And having owned Jtm45, 50W Plexi, and Jcm800's in the past, I do have a reference point to compare to.