Brora-Helmsdale is very nice too. For some distance the train appears to be on the actual beach. I recommend it.
Inverness-Tain is mostly farmland. There's the Clachnaharry swing bridge, and some brilliant views of the Beauly Firth. Then there's Ben Wyvis.
From Tain to Ardgay you skirt the southern side of the Dornoch Firth. There are some lovely views. North of Ardgay to Lairg the scenery gets more rugged. You cross the Firth (now the Kyle of Sutherland) at Shin Viaduct.
Lairg to Rogart is more rugged too, but less sheltered IIRC. I remember there being lots of heather from my last journey.
West of Rogart you head along the northern side of Loch Fleet, with more lovely views. Golspie to Brora has Dunrobin Castle, residence of the Duke of Sutherland. The station building there is a Grade B listed building as well.
Brora to Helmsdale is on the shore, as I've said, with lovely views once again. Make sure you're sitting on the right hand side of the train (travelling northbound) to get these views. Helmsdale-Kinbrace has more rugged scenery once again. Kildonan station on this stretch was once famous for a presumed gold presence in the local streams. I've also seen stags on this stretch before - they love the lush grass. Forsinard has the RSPB Flow Country reserve.
North of Kinbrace/Forsinard is the Flow Country section, which has already been mentioned. The scenery reverts back to farmland east of Scotscalder.