Guess my impressions are a fair bit different to yours, Puddycat! Just shows different people like different things. Most people love or hate Batak. The central square (all cobbled) has a nice pavement cafe, a bank etc. Sitting there watching the kids play is great. The streets are narrow and it certainly has a feel of an old rustic town. There are several museums and also a more modern town square which is huge but a bit "soviet" style. There is a lovely blue domed church (I can see the dome from my house) as well as the older one famous for some of Batak's more gruesome history. It has unusual houses (for Bulgaria) with interesting architecture, all very close together like you get in many old northern English towns.
Some of the other small towns in the area I visited were lighter and brighter, but to me they had no depth and no soul. But I'm the sort of person who loves the historic places like Nessebar but hates the brash tourist places like Sunny Beach.
You might like to know why I chose Batak, apart from liking the place. I think you and I are thinking along similar lines. It's about 4 miles from Batak to the ski slopes, and a very cheap ski option last time I looked. £5 per day for all in, including the ski school. That has GOT to be a great selling point which no other town near there can boast!
The view down the slope looks towards lake Batak, which is gorgeous. There is also things like canoeing on the lake in the summer, restaurant etc. The other towns were not quite as near as Batak. Very roughly speaking, Batak falls within the triangle formed by the 3 big ski resorts (bansko, Borovets, Pamp) and I reckon all will be reachable for a day trip. Plovdiv is not too far with all it's history.
But the other thing was the ease of getting there. You just head east along the motorway from Sofia and then take a main road south and you reach Batak before too long. Under 2 hours I think. The other towns, you would use the same route, but would continue through Batak so the journey will be a bit longer. Apart from Rakitovo, you have to go onto smaller less good roads. Same thing with going to Plovdiv, Batak is nearer. So even Sofia is good for a day trip, because of the closeness of the motorway.
As for Velingrad, personally I don't like it much. I find the architecture all the same and very boring. But as you say, it's still a good advertising point to say you're near a spa town.