1) The humble BMC A-series engine. OK, in my case, specifically the 998cc Cooper variant with twin SU and a much nicer cam.
2) Napier Lion W12. A brilliant bit of engineering with a heck of a lot of history behind it. Many of the "bog standard" ones ended up in various military aircraft (the MoD effectively bought the entire production run), but then you've got a couple of different racing versions as originally developed for the Schneider Trophy air races, and then used in the Brooklands Napier-Railton, the original Bluebird LSR car and John Cobb's Mobil Railton Special - the latter being loaned to Cobb by Betty "Joe" Carstairs, who used them in her powerboats. I got an appreciation for these working on the drawings for these while cataloguing part of the contents of the old Brooklands drawing office a year and a half ago.
3) BRM V16. An interesting and brilliant concept, but late to the party due to reliability issues, and then F1 went to a 3-litre formula shortly after anyway, so it became something of a white elephant. They've re-built a couple from the original blueprints. Owww, my ears!
4) TVR V12, as fitted to the Cerbera in the early 2000s. I have an abiding memory of standing next to Bobby Verdon-Roe's car in the paddock at Brands at a BGT meeting. And then they started it up. It's just so damn visceral, and the noise just makes your bones shake.
5) Ford Cosworth DFV. Don't need to say much about this one. It is THE definitive (and winningmost) Formula 1 engine in history.