Juliet will be along soon to sing the praises of the Hebridean. Like the Icelandic, they have a double coat so you can separate the two and the inner is supposed to be very soft. Haven't tried it myself, mind.
Or you could cross something with a very soft fleece onto ones with good colours - Teeswater, for instance, makes a super cross on a Shetland, or presumeably would soften the fleece of Ryelands... Or even a Blue-faced Leicester would impart softness and lustre?
Fascinating subject - do think aloud here, please!
No, I wouldn't really recommend Hebs as first choice for softness, even taking the time to separate out the top coat from the undercoat. There are much easier sheep to deal with. Heb can be very soft, but only when compared with other Hebs
In days gone by when I had my fleece flock - partly home-bred wethers, partly interesting ewes - I played around with different crosses. We had three tup breeds: Heb, Shetland and Jacob. We had various ewes, some pure, some crosses. The cross with the most amazing fleece was my 'Spinning Jenny' ewe who was PolwarthxDorsetxRyeland, covered by either a Shetland tup, producing white lambs, or a Jacob tup, which produced the most wonderful mainly black, but unbelieveably soft, spinning fleeces. I also put both these tups over a Gotland (not at the same time of course
) and got some lovely, saleable and spinnable fleeces from them - some were black, some were white, one was katmoget. These fleeces weren't as crimpy as Spinning Jenny's lambs, but were a bit lustrous. I think you are unlikely to find lambs of good fleece crosses for sale, because the few people who breed them will be keeping them for themselves.
For a purebred sheep I really think you can't go far wrong with Shetlands, which come in just about every colour under the sun. You need to be picky when choosing your lambs, especially if they are wethers (the best will have been kept entire).
Or, you could have a variety of different ewes, including a Shetland, a Gotland and a coloured Ryeland, maybe an Icelandic, and a Shetland ram for a couple of years, then swap him for a Jacob, very carefully selected for a good fleece (many Jacobs have horrible coarse fleece, but there are plenty with good spinning fleeces)
You can have a great deal of fun trying out different crosses, with the big advantage that you can eat any lambs which don't produce what you want. The first year's fleece is nearly always good, but then disappointment can come with the second fleece, once the sheep is more mature.