If you actually want a mudguard, get a Shockboard or similiar, crown-mounted guards make it obvious how bad most of the alternatives are. Shockboard’s mounts are terrible but the mudguard itself makes up for it.
If you want a bit of mudproofing but you don’t want a mudguard, then the Bender Fender is ace- mine stay on the bikes year round, except when the Shockboards go on. Neoguard is just as effective, but with some drawbacks.
And if you don’t really want a mudguard, and don’t actually care if it works, then a downtube guard. You’ll still get just as muddy but at least you’ll be able to tell yourself you tried 😉 (ideal if you want to complain on internet forums about mudguards being useless)