Vitamin C, you need it in sufficient quantities as not to get deficient and interfere with collagen synthesis. Moderate amounts of fresh fruit and vegetables will see you well covered. Extra supplementation is not helpful, no matter the belief of Linus Pauling. He has no evidence for that his megadose vitamin therapy is helpful at all. The problem with large doses of vitamin C is that it gives you diarrhoea, and may lead to kidney problems. It also increases absorption of iron, which can be a good or a bad thing. Either way: It's well enough absorbed either way. Less acidic variations may be useful to those with sensitive stomachs, but you'll get your vitamin C either way. Fruit and vegetables is still the way to go, though.