DO 20/- From a Demilich comes to mind immediately. I can't think of any DR higher than that.
Tarrasque is only 15/epic but fastheal 40... in smaller groups/low attack numbers this might be better...
Why is the tarrasques regeneration worse than fast healing? Nothing can beat his regeneration so it's like fast healing save that it can bring him back from "death"
You've literally answered your own question.
My bad. It has always bugged me how DR seems to cap around 15-20. I have a PC that can do over 260hp in a single hit, he can kill virtually anything published in one round (mythic, spends a point, gets a second Vital Strike).
I wanna see stuff with DR 30, 40, 50!
Or multiple damage reductions, like, take half damage from slashing, 1/4 from bludgeoning etc...
Of course, this nerfs melee/martials, but the tarrasque should be tougher. :D
Tarrasque (i.e. all Spawn of Rovagug) has DR 60/epic in hibernation mode, iirc.
According to the PRD, you can't kill it... in 3.5 you could finish it off with a wish, in Pathfinder, no such luck. :D
Still, I find DR values to be too low on higher CR stuff. I think DR values should be around 2xCR or more.