That is a very difficult question.

In Economy and Premium Eco, I would see Emirates A380 clearly ahead of Singapore Airlines. SQ has the better crews in my experience, but in Eco the hard product counts more for me and there I find the seats and Emirates ICE much better.

In business class, SQ has excellent food and a strong product on long-haul routes, regardless of whether A350/A380/B773.
At BR you have to fly B787 to get the best, at NH the B773. With LH alegris A350 ex MUC.
If I were to look at the question of who is the best airline, I would always base it on the standard product. I don't care if a airlines have an outstanding product if its just in couple of planes flying on routes that are not interesting for me.
SQ has a strong business class on all long haul routes. In my opinion the combination of seat, crews and food is very strong.

I've only flown First with LH so far, so I don't have the basis to form my own opinion here.

You also have to differentiate between short and long haul.

SQ is the only airline I know that also distributes hot towels on short-haul Eco, plus a snack and drinks. This service would be unthinkable in Europe / North America and is unique for Asian airlines.