The only three I've travelled on are the IOW, Bo'ness and Strathspey. Not been on the IOW since I was a kid and hardly remember it, but I've been on the others this year.
The Bo'ness Railway was very nice. I got Morayshire who is a fantastic loco and the first Steam loco I've been behind in many years, and 26024 who is my first ever diesel loco that's not a HST (and even then, only ever been on ONE HST! O_O)
The Strathspey Railway and it's history has really captured my eye, and I'm not just talking about the railway that's there today, but the complete Highland Railway route from Aviemore to Inverness via Forres. I love how most of the old trackbed of it and at least one of it's branches is still easy to follow on Google Maps and am hoping to get the chance to walk it someday. When I actually did the railway in July, I was lucky to get CR 828 as my loco.
That said, I think my vote goes to Strathspey. The scenery is simply breathtaking, and I wasn't sitting down all the time (well, I was for the way to Broomhill, but not coming back!), which I was for most of the Bo'ness line. My one complaint about the Strathspey is that when I asked for a cup of coffee, I expected just that - a CUP of coffee, but I got a tray with a pot, a delicate cup and other stuff, which was not easy to keep steady while navigating some BR Mark I's under Steam power. I know it's not historically accurate, but a cup would've been easier and safer.
I only wish I got to spend more time on the Strathspey, if not for the Steam fair, but I had to get back to Aviemore for around 2 pm for the next train to Inverness. What's this at the end of the Steam fair they had all their steam locos coupled together passing through Boat of Garten station? Must've been one heck of a sight!
The Bo'ness line has it's major ups too. The viaduct between Birkhill and Manuel is impressive, and I loved being at Manuel watching Morayshire run round and seeing main line trains pass by on the E-G line. Although next time, I'll be sure to get some more air on the journey (and by that, I don't mean leaning right out of the window like a complete idiot).
Can I also just say that I really wish Hornby would make Morayshire or CR 828 as a model? Preferably the former.