The most scenic route is probably via Bathgate & Airdrie. The middle bit is fairly scenic.
I prefer the long way via Carstairs for scenery. There's a few nice bits on that part of the WCML, although they're few and far between. The slow bit around Carstairs gives you an interesting look at the State Hospital if you're into that sort of thing, but I do like the ride from Carstairs to Kirknewton. Very nice peaceful countryside that gradually builds up as you close in to the Capital. Not as dramatic as elsewhere in Scotland, but very nice to see nonetheless. And, as Glenmuthckin says, actually really good views of the Forth Bridges.
But, to be fair, I'd say they're all about the same.
The main line is closed at the moment, but you get some nice views of the Campsies heading North and the bit between Linlithgow and Newbridge Jn is quite nice over the Winchburgh viaduct.
Airdrie-Bathgate has some nice scenery between Airdrie and Bathgate itself, but it's not spectacular. Airdrie-Glasgow is particularly dismal - there is quite a bit to look out for if you're interested in old railways, but otherwise you're trundling slowly through some of the most deprived parts of the East End. And it's every bit as scenic as it sounds. There's an abandoned trailer park, brutalist blocks of flats, cooncil hooses, the M8...
I've been on the Shotts line, but it's been many years ago now. I'm actually cue to use it tomorrow, but probably won't be able to comment on scenery as it'll be far too dark! The fact that I can't remember much of the scenery probably isn't a good sign.
If you can try them all, I'd recommend that. I'd choose via Carstairs.