During Covid, I went down a real rabbit hole with acquiring and flipping gain pedals.
My friend of mine who is a sound man and not a serious guitarist gave me his original Green Russion big box Big Muff. Not really my thing but I was amazed by how my Kemper behaved with that unit in front of it. It felt totally alive.
Some of the overdrive pedals, I have gone through: MXR distortion +, original TS9, Rat 2, Walrus Ironhorse, R.A.T. TACK, ARC Effects Klon Klone, original Timmy, Barber Burn Unit, Van Weelden Royal Overdrive, Honey Bee, Lovepedal ZenDrive, Vertex Ultraphonix, Ultraphonix MK2, Hotone Grass, Purple Plexi, Les Lius, High Power Tweed Twin, Joyo American Sound (Tech 21 Blonde copy), Joyo AC-Tone, King of Kings (Not recommended KOT clone), Rockett 45 Caliber, Blue Note, Box of Rock, Weehbo Plexface, Cornerstone Gladio.
I prefer gain stages using only low amounts of gain from each pedal.
When employing primarily Blackface/Brownface clean sounds I used nothing but the Arc Effects Klon Klone and a Timmy for about a year. - The Klone primarily to warm up the sound and the Timmy as a boost.
Eventually I started using warmer Tweed cleans for my basic sound and didn't use anything but the Kemper; I used the treble booster more as an EQ pedal, then I would have Green Scream and Mouse for different amounts of gain within the Kemper. - but more often I would just switch to a different higher gain profile.
I think this is what makes the most sense with the Kemper as it adds many advantages of simplicity and quietness to the rig. I would just recommend using different profiles and tweaking the existing Kemper stomps. There is tons of depth there if one is a dedicated tweaker. - but it isn't what I currently do.
For external gain, I currently go Vertex Ultraphonix > Rat 2 > Timmy into a custom warm Tweed clean sound
The Vertex gain is less than 9:00 and acts more light a smooth boost or compressor, adding almost-clean sustain to my sound. I love this. I step on the Rat which is at around 8:00 (gain almost not engaged) to add a midrange that makes my sound 'push forward' a bit and is better for rhythm, then I have the Timmy just as a 'more' button not really using its gain but as a boost that also enhances tremble.
Synopsis: One essential pedal?
Blackface core sound - some kind of Klon (I think the Tumnus version with the 3 band EQ is one of the best)
Tweed core sound - Ultraphonix (original not Mk2)