I would suggest it depends on various factors - do you only want scenery, or is railway interest a consideration? I may have been unlucky with both the Shotts line and the Falkirk Grahamston via Cumbernauld route, but mostly I just watch soggy fields!
Falkirk High line: plenty of railway interest during exit from Glasgow, good views northwards to the Forth and Clyde Canal and to the hills to the north – the Kilsyth Hills, and then the Ochils beyond Stirling. Pleasant countryside right through to the Edinburgh City boundary, apart from the industry (and good views of Edinburgh Airport) at Newbridge. Linlithgow Palace is probably the best-known landmark on that stretch.
Cumbernauld and Falkirk Grahamston line: not remarkable at its western end, but better after Cumbernauld. There is a very nice wooded stretch with a waterfall in Cumbernauld Glen (visible on the south side of the track, when looking westwards).
Airdrie – Bathgate line: Glasgow/Lanarkshire urban sprawl to Caldercruix, but lots of interesting railway features to enjoy if you like spotting relics of how it all used to be. Pleasant but unremarkable moorland scenery after Hillend Reservoir, and then an urban section through Bathgate and Livingston with a further few miles of farmland before reaching the E-G via Falkirk High line.
Shotts line: another one that is more pleasant at its eastern end, particularly after Kirknewton IMO. Western end has a lot of railway interest as it departs Glasgow Central, but the endless urban sprawl tends to get boring once off the multiple tracks!
Motherwell and Carstairs: haven’t used this for ages, but recall it is very pleasant scenically once you pass the Wishaw area. Of them all, this is the one I would prefer, partly because it is difficult for me to justify a trip on it (given that I stay on the northernmost of these routes!)