It is the father, Angus Tuck, who seems dearest of them all.

Winnie looks around at the Tucks after spending her first night with them, and realizes she loves them all.  She glances at each in turn, "and then her eyes (go) to Tuck and linger on his sad, creased face...it occurr(s) to her that he (is) the dearest of them all, though she (can't) explain why she (feels) that way" (Ch. 18).

When Tuck first meets Winnie, he makes her feel "like an unexpected present, wrapped in pretty paper and tied with ribbons" (Ch. 9).  But in addition to the obvious fact that he truly cares for her, I think Winnie is drawn to him most because of their conversation on the boat.  Tuck wants Winnie to grasp why it is so important that magic waters be kept secret, and his reasoning shows a depth of understanding that the others do not have.  He longs for the opportunity to die, for with that comes the privilege of growth and change, and of being part of the natural cycle of life.  He lives with the sadness of being "stuck so's we can't move on" (Ch. 12), yet his main concern if the secret gets out is not for himself but for others. Tuck knows that the idea of eternal life on earth will be appealing to most, and that they will discover its tragedy when it is too late.

I think Winnie instinctively feels closest to Tuck because he is intelligent, loves and feels deeply, and is genuinely concerned for the world beyond himself.