Rodinal.
For many years I used a proprietary developer whose formula was not published (Johnson's Unitol, if anyone is interested). When it was taken off the market I was suddenly forced to make a change. At that point, I decided to insure against a repeat performance by selecting a developer whose formula was known, so that I could carry on if the worst happened.
My criteria had little to do with performance and everything to do with convenience. I had had limited experience with developers in those days, and solely with 35mm film. The only developer I tried that gave a noticeably different negative to my eyes was Acutol (an acutance developer, as the name suggests). The first film I processed with it was a revelation, and I still recall the way it rendered the stone of my village's church. The downside was that grain was larger, and I have always disliked grain. The only thing in black and white photography that I hate more than grain is the advice/hints so often given in magazines about grain being a possitive advantage, and something to be sought after and greatly prized in black and white (often together with a soot and whitewash contrast). But I digress.
The result was that I could see advantages to acutance developers, especially if grain wasn't going to be an issue. My second level of desirable features after a published formula was convenience - I wanted a liquid rather a power - and a good shelf life. Acutol didn't have a good shelf life. Rodinal fitted the bill, and I settled on it. It works well with 120 too, in my experience. I've never put a 35mm film through it though.