Better to avoid
Surprise: no. Despite every cartoon ever made, most adult cats are lactose-intolerant. A saucer of milk commonly means diarrhoea a few hours later.
Kittens digest their mother’s milk with the enzyme lactase, but production drops sharply after weaning — most adult cats can’t break down the lactose in cow’s milk. Undigested lactose draws water into the gut and ferments, causing diarrhoea, gas and discomfort. Cats love the fat and protein taste, which is why they beg convincingly. Cream is slightly lower in lactose but much higher in fat; neither belongs in the bowl. Kittens should never get cow’s milk either — they need kitten milk replacer if not nursing.
Best: none. Some cats tolerate a teaspoon or two of milk without symptoms, and lactose-free “cat milk” from pet shops is a safe occasional treat (still calorific). Water is the only drink cats actually need.
Lactose-free cat milk (pet shop), or a few licks of water from a tuna can (in water, not brine) as a flavour treat.
Related: Can cats eat cheese? · Can cats eat tuna? · Cat Age Calculator
This article is general information, not veterinary advice. If your pet has eaten something potentially harmful or shows symptoms, contact your vet or an emergency clinic immediately. Full disclaimer.