This article suggests that 75% of the landing are on the E-W runways and I've only seen take-offs and landings headed west, on them. For those runways, the city will be on the north side of the runways, or starboard side of the plane. The most visually spectacular landings would be on the N-S runways, landings being normally being south facing. So, again, you will want to be on the star board side of the plane. The best conditions are when the sky is clear of smog and it's night time.

IIRC, most of my flights into LAS enter the area west of Las Vegas, fly east over the area south of Las Vegas, then circle west for landing, usually going as far as Lake Mead. During this time, you would get your first glimpse of the city, port side.

YMMV