Definitely Marc Simonetti these days - Josh was brilliant but Discworld was moving on when he died and so he's firmly ensconced in the past now when the magic was still wild and anarchic and we all, including Terry were exploring Discworld for ourselves.
I like Paul's work and Stephen Player's too, very much (my Great A'tuin coffee table, like the forum turtle is based on the latter's work), but the difference is that they're both highly
illustrative rather than interpretive and it's that which makes the difference and gives out the edge of spontaneity and 'edginess' that Discworld demands. Terry himself once said that Josh's visuals brought his words to life in an unexpected and highly apt manner and it's that element of creative challenge that marks the departure from faithful and thoughtful illustration to true artistic high adventure and Marc/Kemar, like Josh has that gift.
It's going the extra mile in other words - the unexpected fillips like the shadows of the evaporated thieves on the wall in Guards! Guards! and the auditors' 'wrong' skeletal horse for their replacement Death in Reaperman that make all the difference to the imagery and the spirit of the story rather than painstaking accuracy to the telling of the tale and screams out from Marc's 'loose' and almost freeform brushstrokes that are about
impression as well as
expression that brings out the subtleties. Josh's work had that same fluidity and 'bursting out' vividness, but his approach was more brash and manic - 'in your face' style which suited the early Discworld in a gutsy, raw way that Paul and Stephen have refined and pared down to some extent so it becomes more recognisable and real to the point where it's, not lost the energy so much as put it under constraint somehow? Marc has let the wildness break out again and whilst he's not always successful (The Truth cover work seems rather tame for instance) there's always something there that will scream out at you that's so 'right' even if you didn't 'get' that nuance in your reading - there's more to illustration than the detail in other words. You need to spill the guts of it to capture a truly original spirit as well...
