Because of people's concerns that you can't compare between airlines, I'm limiting my response to widebody flights I've taken with CX within the past two years. My vote is the 777-200. I know that this is a belated trip report too, but because my opinion of which aircraft is the most comfortable is based on actual trips, please give me a bit of literary license here and don't report me to the trip report police.
Jan 1999 777 HKG-BKK-HKG. This was my first flight on a 777. I think it was a -200. I was seated next to a window right behind the starboard engine and was impressed with the power and low noise levels. I remember enjoying the FAST ascent to cruise altitude. That was my first time in a 777. No problems with "fighting elbows" but maybe because my girlfriend and I had adjacent seats and we had the armrest up. I do recall that on takeoff and landing I had plenty of room for my elbows. The pop-down swivel overhead bins looked impressive above the center seats and yes, the plane looked huge and spacious. No PTVs in economy then if I recall correctly, so I had to track our trip on the projection screen. What I liked is that I didn't have to duck my head to fight with the overhead bin when I got out of my seat. The plane handled turbulence very well (it didn't look or feel as if it were going to break apart). Flight was about 3 hours, so that could have contributed to my comfort level. The plane was about 95% full. Leg room was good.
August 10, 2000 A330 SFO-HKG. Got a window seat above the starboard wing. I felt cramped the whole trip. The seat was probably the narrowest seat I've flown on a "widebody". Played elbow tag with the passenger next to me the whole trip. I was impressed with the quietness of the engines, but the LOUD rattling of the central overhead bins upon takeoff really put a stop to my peace and quiet. Luckily the rattling stopped at about 10,000 ft. It took us 45 minutes to get to cruise altitude of 32,000 ft. I don't know if this FL was as high as it could go with a full load, but we did climb to 39,000 ft after we flew over Japan, where the turbulence was less than at 32,000 ft. Is the low cruise altitude early on required to save fuel on the 15 hour trip? I think my flight would have been more comfortable if we had been cruising at 39,000 the whole way. The cabin felt comfortable during turbulence, but the wings looked like spaghetti flying through it. Leg room was good. My overhead space was minimal, and I had to duck to avoid the overhead bin each time I left my seat. The plane was absolutely full. PTVs for each seat, and I tracked our progress/speed/altitude/ETA most of the trip on channel 19. What a great program!
August 11, 2000 747-400 HKG-DPS (Denpasar, Bali). Got a window seat behind the starboard wing/engine. Felt like I had a lot of elbow room and leg room. PTVs for each seat. We reached FL390 about 40 minutes out of HKG. Lots of turbulence, but the Big Mama's wings handled it quite well and I barely felt anything in the cabin. Flight lasted about 5 hours.
August 21, 2000 A330 DPS-HKG. Got a port seat way in the back, just 3 rows before the lavs (68A?). Same rattling noise from the central bins upon take off, but what power from those engines! The cabin noise from the engines at cruise speed was pretty loud, but not as loud as in the CX 747-400 or 777. We reached FL390 in 45 mins, then climbed even higher to FL410!!! No turbulence on this flight baby!. This was my first flight on an A330 (I think it was a -300). Leg room was good, but the seats were just as narrow as in the A340.
August 21, 2000 A340 HKG-SFO. Very similar flight to the A340 trip on August 10, but I'm mentioning this leg because I got seated next to a very large gentleman. That must have been the most uncomfortable trip I've ever taken in my life. I had seat 30A so got a good look at the engines. Very quiet, but that doesn't help you if you've got a Hulk Hogan sitting next to you for 13 hours.