Closest call I ever had was on joining to land at my, then, regional airport base. Close enough to be within the CTR and in radio contact with ATC.
Angling in at 45° to the downwind I was aware of another aircraft returning from the same field as myself but from his position reports I 'knew' it to be about 4, maybe 5 miles behind. Time enough for me to get round the circuit and land, but as I began my turn onto downwind I looked over my right shoulder and saw this other aircraft absolutely head on to me and just RIGHT THERE! I stuck mine on the left wingtip and pulled, dreading the impact that would cut my tail off. It didn't come and I shudder at by how little we must have missed.
My 'orbit' would have been about a mile from the tower, at circuit height, in clear visibility yet nothing was said on the radio, not by ATC, not by him, not by me. On completing the 360 I saw the other machine tooling away on the downwind leg so, very shaken I followed along. He landed uneventfully and a few minutes later so did I.
Back at the hangar I asked "had he seen me?" "No" he said, "I thought you had landed." My reply was rather more colourful but the substance was that he then had no idea how close we had just come.
But, I wonder even to this day, how could that be? We were both on the radio, I had heard his transmissions, knew he was following, thought I knew where he was, and he could not have heard me receive a landing clearance nor my read-back, for neither of these exchanges had occurred. Was he deaf? Was he blind? How did he not see me? I was in his 'gunsight'! It's true that my aircraft is quite slender when seen from behind but it's not invisible and he was close, boy was he close. And how did he miss seeing the thing roll nearly vertical and pull away right in front of his face? What was he doing? Where was he looking?
It's hard to objectively assess but I believe that I was within about 1 to 1.5 seconds away from being centre punched by that aircraft that day and had we hit there is no doubt in my mind that neither of us would have survived.
He still flies, as do I, but if I know he's in the air I take care to keep out of his way. I believe he is, to this day, blissfully unaware of how truly close we came that day.