I know this is sacrilegious but I've always been underwhelmed by Ditko covers. They always lacked an 'oomph' of some sort.
He was a great storyteller though.
I used to feel this way as well. But I think Ditko is an acquired taste, much like the musical genius of Jimi Hendrix, it is hard to grasp on the the first go round.
The covers of ASM #23 and #24 really sold me to his brilliance.
I like those Spidey covers too. But for every solid Spidey cover that Ditko drew (like #3, #23, #24, #28, #29) there are plenty of clunkers like #8, #22, #30, and #31. I can't imagine how Marvel let the cover for ASM #30 ever go to the printers.
What's amazing to me is that the stretch of Romita covers that followed Ditko's work was just incredible. For me, ASM #39 to #75 is one of the best sustained runs of covers I can think of. They're not all great, of course. But I think the number of classic covers in that run is far greater than the covers that are just ok.