The really nicest places to live are probably Bath, Salisbury and Marlborough, but Bath is pretty expensive.
I live in Devizes, which I love, but it may not be to everyone's taste! It's a small old fashioned town and there's not a lot here. Marlborough on the other hand, is probably smaller but is pretty and has nice shops and places to eat - quite trendy and 'up market' - housing there certainly costs more than most places in Wiltshire.
Chippenham is larger and has lots of amenities, but I don't know it well and I'm not sure it's that wonderful (hope I don't offend anyone here).
Swindon is about the size of Reading - it's a pretty big place and there is everything there, but it's not that nice in my opinion.
Wiltshire is I think the largest English county, but it probably has one of the lower population levels. It's definitely a very rural county and if you don't live near to the M4 or A303 it can take quite a while to get anywhere else! I absolutely love it and when I moved here from Hampshire, which isn't exactly far, I really noticed the slower pace of life here and the fact that people were more friendly. However, I can imagine that it could be quite a culture shock after living in London, as there's not a lot here!
Well that got rather long Hope it helps.