Just seen this thread, fallen way behind on the forum lately!
In terms of recommendations, for pure scenery I'd go for the Rhymney branch as it is pretty decent. I'd also suggest the Treherbert branch too, I've not been up that one since the mid-2000s (there were ex-ScotRail 150s pre-refurb in use at the time, that's how long ago it was!) so memory is hazy, however I recall it being quite nice.
For getting a feel of industry etc, you'll find it harder to find that these days with more and more of former industrial sites being re-developed. The first place that came to mind though is Taffs Well, and for getting a vibe of the Valleys life (bear in mind again this is based on memory from the mid-2000s) nothing quite gives you that feeling like a trip north of Abercynon on the Merthyr Tydfil branch.
One thing that did surprise me when Kite159 and I did a bit of a Pacer farewell a while ago was how little investment in stations there is on the Valleys routes. Places like Radyr have seen a fair bit of investment (that car park and ticket office are a vast improvement on what was there in 2005 for example, and Llandaf is far better now), elsewhere the amount of broken/not-working information screens and in general run-down shelters etc (I'd not want to wait for a train in some of these places!) is surprising. I swear some parts of the Rhymney line haven't seen any investment since Valley Lines existed as its own TOC!
If I wasn't now running low on time to get prepped for a mini-break, I'd add photos to back up my points, maybe later on. More importantly, have a brilliant trip!
