About 10 years ago my dad gave me his 81 BMW 528i automatic, with a 170 cube SOHC six, 175 stock horsepower. About 110,000 miles on it, it still starts with barely a flick of the key, and pulls strong and smoooooth to its 6500 rpm redline. I have had it cruising at 80 to 90 mph for 8 hours, never missed a beat, and has never had any engine repairs. It is well maintained and reliable. Starting to use a bit of oil, but apparently the valve guides tend to need replacement at about 100,000 miles.
I also have an 85 BMW 325e, the "economy" six they made for a while.
A lower reving and torquier engine, with just over 200,000 miles on it. Again, no major motor or power train problems, runs all day at 90 mph if you want to, and gets about 30 Canadian MPG at 70 mph.
I have a lot of respect for the Ford 200 6cyl in my 80 Mustang, but there is no comparison to the BMW motors in smoothness and performance.
Not saying BMW 6's are the best, I haven't had a lot of experience with other 6's, but they are definitely right up there.