I don’t know a ton about foreign motors other than surface level stuff like the 2JZ, Wankel, etc. so please forgive my ignorance outside of US motors.
However, in my eyes, it doesn’t get better than the simplicity and easily available power of the SBC, BBC, and 6BT. What are your all time favorites and why?
BMW M5x/S5x. This includes the M50/M52/M54 and their S counterparts. Essentially the Jeep 4.0 of the BMW world, they are overbuilt from the factory and the ironblocks are capable of handling serious boost with zero internal modification. Very simple to work on, and parts swap between them like Legos which makes budget engine builds super easy. They’re also pretty good on gas for an inline 6. I have an M52B28 in my e30 and I’ve taken it to the track multiple times and also daily driven it for 2+ years without skipping a beat. I can get ~27mpg on average, and my car is shaped like a brick lol. I love them!