I've had a lot of Hornby steam outline pass through my hands over the last ten year's or so, but as a Southerner I do have some bias towards the sunnier end of the country.
The N15, Castle and the Stanier 2-6-4T are just about the best I've seen, they all capture the look of the originals well and run well to boot. I thought about the Britannia too, it too runs like a dream, but there is a balance to strike between the amount of seperate details fitted and their tendency to come off and on that basis the BR7 basically needed stronger glue to put it nearer the top of the pile.
The original WC/BB, Gresley A1/3/4 and Stanier Black 5 are fine, but Hornby have done better. Oddly the rebuilt Merchant Navy that started Hornby on the 'super-detailed' range of today, looks a bit lacking in detail nowadays.
Other than the Gresley corridor stock (too boxlike, see Coach for more details) Hornby have produced some cracking coaching stock in recent years. The best thing about modelling the SR nowadays is having the excellent Maunsell stock to run with the locos, thirty years ago it was Sir Dinadan and some pseudo-GWR stock in SR lined green.
Glenn