I have played original Rat's back in the day, and since Hollywood Blvd, have had a few Rats. I did, early on have one on my board, and gigged an original 81/2/3 or so. I used it as a 3rd dirt channel into the clean on my Marshall VS100R.I also have currently the following:
1. Mooer Black Secret
2. Ikebe Rat2 (white)
3. Proco Solo
All in all, I have to say the Proco Solo is my favorite of all of them, for no other reason than the Flexibility; Hot, Melt and Burn gain modes, and a Tone and Scoop control.
That said, a generic Rat2 is all the Rat anyone needs for anything that needs a Rat.
The Mooer black Secret is really good (as is the Mosky Black Rat). The Turbo mode is nice for those who want to slam a Marshall or whatever.
There are so many good Rat Clones today, can't miss: 1981, Hartman, Katzenkonig, Rattler, Ratsbane, even the Joyo Splinter, and of course, for those who can't decide, the Pack Rat
They all do "Rat" pretty darn well. It isn't a hard circuit to do apparently. And I am a believer in LM308 doesn't mean a thing.