Fort George at Ardersier (Historic Scotland property) or Chanonry Point on the other side of the Firth, near Fortrose.
They come in really close, especially if the tides are right. I can never remember if it's the incoming or outgoing tide that's best, but there's a visitor centre just on the northern side of the Kessock bridge with very knowledgeable staff.
Mind you, we just turned up at Chanonry Point and within an hour we'd seen a mother dolphin and calf, not 50 yards from the shore. Absolutely breathtaking.
There are various dolphin spotting cruises, but do take care with these. Some of them go too close to the dolphins and they're causing real harm by disorientating them.