I do different things depending on the meat.
Roast beef - roast potatoes, Yorkshire puddings, peas or green cabbage, mashed parsnips, gravy.
Roast chicken, roast potatoes, roast parsnips, stuffing, roasted carrots, green beans/runner beans or sprouts if seasonal. Occasionally chipolata sausages alongside, occasionally bread sauce or gravy.
Gammon, roast potatoes, mashed swede, cauliflower cheese, leeks or green cabbage.
Donโt usually do pork or lamb, but sides for pork would be similar to for gammon, might add red cabbage. For lamb, would replace Yorkshire puddings with roasted onions, and peas for green or runner beans.