Good question. I only take vitamins that I know I need and have reason to believe are of good quality from a reputable company. I've never taken cheap vitamins because I fear they would be of poor quality.
I recently resumed taking a B-complex because I don't function well if I don't, have tested low in the past and again recently. I take D3, always with K2, due to testing low in the past. I don't take iron, but I check my ferritin level annually because it's been low in the past. I take magnesium citrate because we're almost all deficient, diabetes or no diabetes.
I take selenium and a balanced zinc/copper formulation because I have a long standing thyroid condition and my whole food multi-vitamin and mineral supplement doesn't have enough of those minerals, but does have adequate iodine. I take the whole food multi to be safe, to insure I have no vitamin deficiencies.
Other things I take are turmeric in the morning to keep inflammation in check, vitamin C with each meal because it's an antioxidant and provides additional electrons when needed. I also take cod liver oil for the vitamin A, CoQ10 for mitochondrial health, alpha lipoic acid to reduce risk of diabetic complications, and a whole food eye formulation because I'm showing early signs of eye disease. Additionally, I alternate between eating omega-3 foods and taking fish oil.
I was just thinking the other day that it would be nice to have a list of the vitamins that are most likely to be depleted by diabetes.
Some people get the vitamins they need from food. Others, like me, do not, perhaps because I have a history of inflammatory bowel disease (IBD) or because I'm a poor methylator. I use lab tests to monitor. Those of you in the UK are so lucky that they annually check your folate, B12, D3, and ferritin levels. I have to pay out of pocket for those tests, but it's worth it. All are important.
I think we each need to supplement based on our health conditions, what we eat (and don't eat), and lab test history. If I didn't have three chronic health conditions and if I didn't feel so poorly when I don't supplement, I probably wouldn't supplement as much as I do.
The need to supplement (or not) is so individual. All that said, I think engaging in regular physical activity, getting sun, fresh air, and good quality sleep, and minimizing intake of unhealthy sugars, starches, and omega-6 seed oils - (canola, corn, cottonseed, saffola, soybean, sunflower, peanut) - is equally important to maintaining good health.
We make the best choices we can based on our needs and resources.