I think my favorite over the years has probably been my Wylde Overdrive (original version), Boss SD-1, MXR Custom Badass Modified Overdrive, Ibanez TS Mini, and JHS Bonsai has some good sounds as well.
Basically, something TS or SD-1 based.
The Rat set to low gain into a crunchy sound can work. Can't be too high gain amp or distortion pedal wise (wherever the main source of gain is), has to be right at the point where you get full signal on the distortion pot, and the voicing of the amp or pedal has to work with the Rat.