[userjack6880]

Making it a multi-checkbox, I had to vote for the top 4 tied.

Boxer Four - not much to explain with this one. In our twins, but also the mainstay of the Subaru brand. Always respected them, and love the concept. But, I'd have to say the twins really made me fall in love.
Boxer Six - similar reason, but mostly because Porsche used them in the 911 for many years.
Inline Six - I find this format interesting, as you create a very narrow and very long engine. BMW used them for years in their 5 series, and the SUV I used to have (GMC Envoy) had one.
V8 - nothing like the deep sound of a good V8. Can't say much more. The Corvette probably has the most beautiful sounding one.