Horsemart will get the most interest, but also the most timewasters.
Can't beat Horse And Hound for genuine interest.
Tried Horsequest once, with no success at all (bad value for money).
Don't know about the others, but Western Daily Press is effective if you are in the right area.
It depends where you are geographically. When I lived in Herts I found Horse & Hound the best. However now I live in Scotland I would always try Scottish & Northern Equestrian first. I sold my last horse just from the last website last time which cost very little.
I agree it depends where you are, i think local papers and papers like the Adtrader are good! But depends what you're selling, if it's something really nice (and expensive) then i think sites like horsesmart and h&h are better.
I've got a horse on horsemart at the moment & think he's very competitively priced, but only had 2 calls in the first week!
A lady told me about Horse Hunter where its free to advertise & she both sold & bought off there. Its very similar to Horsemart.
I've found H&H the best for genuine interest, adhorse seems to be totally useless - one of our adds has had over a hundred thousand hits and not one call or email. I must say horse and hound is pretty expensive though for the size advert and the fact it is only in a mag for 1 week. Have also tried horsedeals - booked a full page and didn't get a single call from them either. I don't like the new horsemart and the fact the pics have to be shrunk down to be able to upload - have no idea how to do that - am not great with computers!!
Bought my mare from Horse Hunter but not many people had been to look at her. My rant is that when things are sold people don't update the ads. I always presume that things in ads older than 10 days have been already been sold so don't reply to them even if they look ideal.