I'm sorry, I didn't read the last few contradictory posts, so I am just posting about mashed potatoes because I love them so much that I was inspired to respond. I have suggested mashed potato variations at the end of my post.
First off, mashed potatoes are good, but I wouldn't rely on mashed potatoes to provide me with a healthy daily amount of C. If you're worried about Vitamin C, eat more greens, citrus and take Emergen-C, or something similar like Barocca (sp.?) + C every day.
I love Emergen-C, so I had to add that. I love Barocca too, but I'm 'merkin so I only get Barocca when I'm in England or some other area where the English did scandalous colonial things. I think Barocca is heavy on the B vitamins, whereas Emergen-C is all about C.
But, back on topic… I prefer my mashed potatoes lumpy (not whipped) with scallions diced very small, and a bit of butter. Sometimes I'll add some milk to make my mashed potatoes more creamy.
I wanted to add that you can vary your mashed potato intake by combining the potatoes with all kinds of other roots: carrots, sweet potatoes, yams, parsnips (surprisingly good, but sweet, so don't over do it), rutabagas (also don't over do it; they'll be too bitter), or, the best of them all, celery root.
Celery root has tons of Vitamin C and is incredibly nutritious and tastes amazing when combined with potatoes. This is basically the reason I responded to this thread. Mashed celery root and potatoes is the best possible thing in all possible worlds. (Aside from plain mashed potatoes.) I can't recommend it highly enough. So very, very delicious.
Just make sure you boil or steam your celery root separately from your potatoes, because celery root is very dense and takes longer to become soft enough to mash. Please try this. You will be very happy that you did. Season it the same way: a little butter, maybe some milk if you're feeling it, and you can skip the scallions, it's good just the way it is. Celery root + mashed potatoes = Mmmm.