Like "what's your favourite album?" questions, the answer changes regularly and, for me, there is definitely a bias towards newer pictures, so a Puffin I saw in Fife recently is on my mind at the moment. In fact, it's my avatar.
For something less monochrome and stark, I also like a Malachite Sunbird from South Africa. That was especially satisfying because my only previous shot of the species had been eight years previously, and it was made on the exact same plant. I sat there, in Kirstenbosch gardens, recalling the previous (much poorer) shot and - for once - the dream scenario came to pass. Hoping for another Malachite, an individual magically dropped on to a pin-cushion protea exactly where I was sitting and let me blast off a number of shots (half a dozen useable) before gradually making its way down the slope via other proteas.
Same bench, same path, same location, same actual bush. Very satisfying, like the kind of positive plot twist that birding so seldom delivers.