The best are probably equillibrium equi-chaps, either the close contact or the hardy chaps.
They aren't cheap though! but do seem to cover more of the leg and stay on better.
Shires do some called called Mud Socks, which are about half the price.
Or Sportabac do some cheap ones, but I don't think they are making them anymore. Friend of mine just bought some made by Castles? which i've never heard of.
And I think Westropp do long neoprene turn out boots too.
If you can't afford the equi-chaps (try ebay) then i think the others are pretty much all the same.
I've used ....
DH...which were very good,
Sportabac...which were not so good although more pricey...
and I've just bought some Castle ones too...(although I've not used them yet, waiting until the sportabac finally drop to bits). They came highly recomended and were about £28 a pair (I think)
Personally I found them to be a pain. I have a pair of the equilibirum close contact chaps, but found that every night I bought my horse in and took them off, I would have to take them home and have them washed and dried to put back on in the morning, otherwise if you put them back on damp and muddy you just spread the bacteria under the boots.
Found it much more manageable smearing bucket loads of Sudocrem on his legs before turning out. Great waterproof barrier, not easy to rub off and antiseptic too.
I dont actually use mine for mudfever....I use them for my twig legged accident-prone boy who every time he gets a cut/scrape/kick on a lower leg it ends up infected and/or grossly swollen.
Plus they keep his legs cleanish ( oh the angst of a grey
)
He's ok in summer time...I think its the gateway fracas at coming in time where he gets his injuries......