If I had to recommend only one vitamin, and that was all that could be taken, it would be B Complex. The B vitamins are involved in more functions of the human body than any other. Plus B complex will provide vitamin C. Wonderful for stress reduction, energy, digestion, etc. Their effect on vitality and energy are probably unparalleled.
Of course, we have access to more, so I will list the ones I have "lived" by for years. Vit. C, find your tolerance level, but the higher the better. I do minimum 3 gr. per day....Vit E, 800 IU minimum per day, (more depending on some things I take)and the B complex I opened with.
I always have zinc on hand, and dose with 30mg. normally per day. If and when I feel the slightest inkling of a cold, or anyone around me has a cold or flu, I immediately up my dose to what some would consider mega doses along with C. I may take 50 mg. every 3-4 hrs. This regimine has knocked more oncoming colds, etc. than I could ever count over many years. If copper becomes an issue, simply add a multi mineral.
All the above are water soluble, other than vit. E. Thus, it is nearly impossible for them to become toxic. Fat soluble, on the other hand is another story. I don't believe folks get enough Vit. D, and there is an ever growing body of evidence to back this up. Vit. A can be toxic in high doses, as well as over 200 mcg. of selenium.
As another note, iron in your multi is something you want to avoid being male. Women on the other hand need iron, menstrual cycle, etc.