What’s your favorite inline-six?

Aston Martin or A jag 6cyl.

You know I gotta throw the S54 into the conversation. 

All the revs–like 8000 rpm from the factory–plus individual throttle bodies. And it just looks good sitting there in the engine bay. 

Just about anything Japanese for me, but this one has a special place in my heart:

3.8 L XKE 6 cylinder.  Just beautiful.  A workhorse six, the Trumph TR-6, really a tractor motor.

Aesthetically, the Alfa Romeo 6C 1750 - 2500 from late-'20s into the '50s.

From personal experience, the Daimler-Benz M103 3.0L, silky-smooth for extended cruising at more than 2X the posted speed limit on the Interstate, I-95 in North and South Carolina.

My vote, based on sound, goes to the Triumph TR6 engine. I had one for a while and with the proper aftermarket exhaust system it was incredible. They don't call them the "hairy-chested sportscar" for nothing. I have had a couple of Z-cars over the years and based on performance they are better but they just didn't sound the same.

i think the the jaguar XK 3.8  is the pinacle of 6 cylinder design 

I second the Jag XK 3.8L. All polished up with the triple carbs in an E-Type it's pure eye candy and sounds good too.

My dad had a Jaguar XK120 when new.  He would always say it was the best sounding car ever, especially because the sound went straight to your gut.  I have had the pleasure of driving around in an XK120, and I understand what he is saying completely.

I find the BMW straight six sounds to be terrible. Someone below says their TR6 was great - but I personally have never experienced a great sounding TR6.

Finally I will say that the best sounding straight 6 I've ever heard driving by was a 240Z with triple Webers, custom exhaust headers, and straight pipe.  It was very much like my dad said about the XK120 - you could hear and feel that vroom straight to the gut.

The Jag engine is an early long stroke design that looks great but doesn't produce much power for the displacement. I have owned several and enjoyed them, by they are by no means a contender for favourite.

The TR-6 engines was a travesty of smog gutted pushrod engines - it had c. 105 bhp while the one built the way the should have been (same engine with fuel injection in the British home market cars) had 150 bhp.

My choice is the same as David mentioned, the S54 BMW six. I own one in a Z4M coupe and it is amazing - 7900 rpm (8,000 in the M3)  with 338 bhp and one of the best sounding straight 6 engines I can remember.

There will be some who tout the Toyota 2JZ engines and they are indeed admirable units - similar to the S54 except that they 'cheated' and had to use twin turbos to get there. They are, however, immensely strong and can be easily made to produce silly power levels (- 500-700 is easily attainable for street use.