Well hard to tell; there is 3 men above all Tarres (SSDT win, bikedesigner, coach (Raga) and biggest contributer to modern trials riding techniques). Dougie always solid and on top indoors/outdoors/SSDT etc (never does any mistakes), Bou dominating everything and mastering the 4 stroke never tried SSDT. Funny is that Dougie was riding worse on the 4 stroke and Bou really stepped up from being incosistent and 5th to 10 strait titles...
Miller and Jeff Smith was another era so hard to compare. Less international competition in trials.
Regarding SSDT. No one wins SSDT first year a top 3 is great and 5 would be good for a top 3 WCH rider.Top 10 realistic if no preparation. You need at least one year to learn the event. Bou with his smooth riding style would surly win in 3 attempts with decent preparation (some straem training up in andorra and not only dry rocks) unless struck by bad luck or ignorence of making a seroius attempt or Dougie or someone else having a monster week... Look at Pons, Corgot, Bilbao Spaniards taking the win.
Regarding Enduro I'm pretty sure both Dougie with his determination and Bou with his smooth but attacking riding style would have taken WCH medals if they've gone for that career. Merriman, Salminen,Paul Eddy, Knight where all X trials riders dominating the sport.
However the most impressive lap ever must be Dougies 2nd lap on 2nd day Hawkstone Park 98. Non stop ice road slippery mud. Jarvis did a great lap 30; Colomer leading but 48. Dougie 4... !!!! WTF some of the sections were inridable for less than a lucky 3. Also Bou in Spain 08 and Tarres in Sweden 87 have been my personal highlights of their careers.
Conclusion shared 1st place for Dougie, Bou and Tarres.