I've not flown Y internationally since 1999, but prior to that my best longhaul Y experiences were:
BA HKG-LHR in 1996 - on a 747, the forward Y "mini" cabin. One lady and I had middle 4 between the 2 of us, so very comfy. The stewardess (I refuse to use the modern expressions) working our section was HK-based and absolutely charming. Kept the wine topped up during meals, offered plenty of minis of JW black after dinner, regular offers of water / tea / coffee, chatted with everyone who was up for a chat about anything and everything. Totally customer service focused and appeared to enjoy delivering top notch customer service. The flight flew by and nearly everyone who'd been looked after by our stewardess warmly thanked her on disembarkation and quite a few remarked how terrific she was as we walked up the gangway.
LH FRA-SYD in 1987 - on a 747. Again, three seats between 2. Terrific and warm customer service and a comfortable flight. The cabin crew and I chatted in my schoolboy German (which after six years of stern and disciplined instruction from my German teacher was actually pretty good) and everyone felt very well looked after.
KLM MEL-AMS in 1998 - on a 747. Had exit row with crew jump seats in front. Great, chatty crew who were keen to look after everyone. Was on a choir tour, so the crew made us perform to the cabin and "paid" us with champagne after the performance. Very, very jolly flight.
Also had some nice flights on QF in the early 1990s on the 747 SYD-LAX in the 2 seats at the rear of the cabin
Worst Y - Ryanair Luton to somewhere in continental Europe in 2001. Everything was simply ghastly! I vowed I would never step foot on an LCC ever again. I managed to keep true to that vow until bloody QF ditched MEL-NTL and replaced it with Deathstar. I have to travel that sector sometimes. O vey.
A dishonourable mention goes to Philippine Airways SYD-FRA in 1987. From memory it was a DC10 SYD-MNL, then a 747 MNL-AUH-FRA. As we landed in FRA I watched part of the skin of the wing tear off.