The best view is subjective, and depends on the runway in use (wind direction).

Weather in Thailand now is pretty much changed to NE monsoon, so approaches will be onto runway 01 (rather than 19), so sitting on lefthand side (might) show the city once established on final approach - but the view won't be great.

However, since you'll be approaching from the north initially, I reckon that being on the right-hand side might be better, although you'll be higher, and the city further away. If landing on 19, better city views are had on the right-hand side to touchdown.

Sydney - as above (still depends on runway in use.). My fave seat on A346 is 02A, which would suit your purposes nicely.