Of the mods I've had (most by now), I think the most versatile is either the JF XTC (clean and Marshally in Plexi mode, Blue, Red, etc - as described above for the SG XTC), or the Salvo 50R (It does cleans to serious brutality, all with just a twist of the gain knob, and sounds great everywhere - the Graphic is close I suppose, but a different flavor, and doesnt do cleans near as well IMO). Honorable mention to the Salvo Stonerverb - also great clean to brutal, but again, of a different flavor.
For my tastes tho, if I could have only one, it'd prolly be the VybeDeluxe. Does beautiful, warm, spanky, and/or sparkly cleans to very thick low/mid gain to convincing Marshall mid-gain, and even some pretty **** good high gain stuff (can get kinda Recto-ish) - all just in the Vybe mode (which is where I keep it). All this can be had via dirt pedals out front with gain static. My Vybe + a TC Nova Drive is all I'd really actually need... Plus, the Vybe takes effects in front better than anything else I've ever tried - I can saturate it with 'verb, delay and/or modulation fx and it really takes them as if theyre in a loop. Nothing else comes close in that regard - no idea why, must be some sorcery, lol.
The Twox is very similar to the Vybe in it's versatility, but has a different sound/feel - also a great mod!