Avoid Dubbin
Nikwax is good - especially if you put it on with your fingers rather than a rag as it warms it up and it soaks in. Have drenched boots in the past with warmed liquid Nikwax to get it deep into the leather - stand them in a warm place too, do the seams and welts with a good application of solid Nikwax and then buff. Top up with eithe the liquid or the solid. For extra waterproofness use the solid on the top and do not buff back. Used to recon that a first application to a pair of waling boots would use up a tin or two of liquid and a tin of solid to get a good job done. Mind, that much solid Nikwax applied over the course of a weekend, warming the tin, using fingers, keeping the boots in a warm place produced well proofed boots but they look shit...
R