What I think:
When brewed 'strong', i.e. for more than one minute, good-quality British-style black teas (e.g. Taylor's Yorkshire Tea
) work best with a touch of milk to take the edge off the bitterness. Cheap black tea brewed 'strong' is like mud but palatable if weak without milk.
Most other teas and herbal infusions are somewhere between crap and horrid if served with milk. Some of the stronger chinese teas, e.g. Keemun (aka Prince of Wales) are ok, tho. I'm glad I discovered all this in my student days.
Soya milk is at least as good as cows milk, something about the grainy texture, I think. It's especially good in Rooibos