A few favorite directors:
F. W. Murnau and Fritz Lang, the two aknowledged masters of German Expressionist Cinema. For Murnau, look at FAUST, NOSFERATU, and SUNRISE. For Lang, try METROPOLIS, and the two part DIE NIBELUNGEN.
Sergei Eisenstein: one the giants of cinema history. Known for his theories and practice of montage, but his absolutely unique compositional sense is a joy to behold. A big influence on Welles' style from OTHELLO onwards. Look at ALEXANDER NEVSKY and IVAN THE TERRIBLE PARTS I & II.
Josef Von Sternberg: before Welles, perhaps the greatest visual stylist in cinema. The films with Dietrich are still astonishing pictorial triumphs. Try BLOND VENUS and SCARLET EMPRESS. Dietrich's performance as Tanya in TOUCH OF EVIL completes what Von Sternberg began.
James Whale: masterfully transmitted the silent German expressionist style into the Hollywood sound era, combining it with a wicked black humor. BRIDE OF FRANKENSTEIN, THE OLD DARK HOUSE, and INVISIBLE MAN are essential viewing.
Roger Corman: yes, Roger Corman. His Edgar Alan Poe series for AIP is visually stunning and fascinating to watch. Most were shot by Floyd Crosby, who was DP for Norman Foster on the MY FRIEND BONITO segment of Welles' IT'S ALL TRUE. Try HOUSE OF USHER and MASQUE OF THE RED DEATH.
Mario Bava: a great DP and a great director, Bava managed to create a flamboyantly expressionist color visual style that has been hugely influential on many of today's best directors. A great pity that Welles, who seems to have been creatively stymied by color, never worked with Bava.
Terence Fisher: his beautifully controlled, understated style, combined with Bernard Robinson's incredible production design and Jack Ashers's outstanding experimental color cinematography, revolutionized fantastic cinema at Britain's Hammer Studios in the late 50's, early 60's. BRIDES OF DRACULA and HOUND OF THE BASKERVILLES demonstrate that unique look at its best.
Dario Argento: Mario Bava's successor. His films are very violent, but push the envelope with visual experimentation. Try SUSPIRIA and INFERNO.
Nicholas Roeg: DP on Corman's MASQUE, and co-director with Cammell of PERFORMANCE. Interestingly, admired less for his photography, than for his one-of-a-kind editing style. Try DON'T LOOK NOW and THE MAN WHO FELL TO EARTH.
Ken Russell: the wild man of 70's cinema. The only director who could conceivably do a life of Welles on film. Try THE DEVILS and WOMEN IN LOVE.
Federico Fellini: another giant who needs little introduction. Try 8 1/2, which looks like nothing else in the movies, and SATYRICON, which does the same in color instead of B&W. Fellini's segment of the multi part film SPIRITS OF THE DEAD, is, in part, an homage to Bava. Welles was supposed to contribute a segment to SPIRITS, and wrote the script with Oja Kodar, but ultimately did not participate.
I must confess a lack of affection for, and interest in, the French New Wave. Those films never did anything for me. I much prefer Italian Cinema - Fellini, De Sica, Visconti, Rossellini, Leone, etc. There is a visual richness there that I find completely lacking in the Nouvelle Vague.
P.S. THE TRIAL, along with TOUCH OF EVIL, is my favorite Welles film, too.