SQ 19 from LAX to SIN is flown in a specially configured Airbus 340-500. The seats have some extra pitch, greater recline and even a uniquely designed cabin lighting system. If you can get a seat in Y for a decent price, this flight can be the best value for an economy seat anywhere. It's almost 9000 miles. Worth paying more for in many people's opinion, all other things being equal.

But perhaps the most important thing for you, since your stated purpose is to earn miles, is to watch for which fare classes within economy class (the V, W, Q, Y sort of fare codes) will earn you miles from SQ. SQ is stingy about mileage accrual. Many fares earn you 50% or even nothing at all. So learn your stuff before booking.

It matters greatly which airline you are flying and to which airline you would like your miles to accrue.