My favorite would be the cover for 'The Missing Three-Quarter'. Norwood Builder is close except for that HAND print.
At least you didn't say "Collect the whole series"; that would be out of my price range. I've always like the EMPT one with the very Japanese-looking Alp in the background.
My favourite is the 'Abbey Grange' cover. Whenever I look at it, the plot comes back to me: how Holmes succumbs in the beginning and then overturns everything using only observation and deduction. Love that story and so I guess subconsciously I love that cover the most.
Mine is BLAC I guess, with our local connection and society.
Sitting on the pillows in a dressing gown will always be my favorite. He looks so cool they don't even have to tell you what the story is.