I was torn between Digory and Lucy.
Digory got to see Narnia being created. He got to fly on the winged horse, he got to pick an apple from that special garden, and to plant that apple to grow the Tree of protection. And he got an apple of his own to heal his mother.
Also, he had the Pevensie children stay with him, and eventually got to talk to them about Narnia ...

But I voted for Lucy after all. She has such a special appreciation of Narnia. She was the first of her sibling to go there, she had her special friendship with Mr Tumnus, and in the last book, Tirian sees that she "was drinking everything in even more deeply than the others". And she sees Aslan more often than the others do. Yes, I choose Lucy.

After a lot of consideration, I picked Lucy. She is the closest to Aslan and she gets to go to Narnia the most (not counting Edmund, but he's a boy )
Even though she's not my ultimate favorite character, I think her experience is the best. She goes through sooo much. She has the most faith.

This is a hard question but I pick Eustace he got to sail to the world's end on the Dawn Treader travel to the north and underland and fight in the last battle of Narnia.

Probably Digory. It would be so cool to be able to be present at the founding of the world!

I chose Jill, but it really depends.

If I'm just picking what I would like, then Lucy's an easy choice because she, out of all of the Friends, seems to have the most happy times in Narnia. (Teatimes with Mr. Tumnus, living there all of those years as a queen, being part of the Romp, et cetera.) Among the Friends of Narnia, Lucy has the closest relationship with Aslan and with the world of Narnia, and that's appealing.

However, if I'm trying to pick the one experience that would build the most character (which often seems to be the point of drawing children into Narnia in the first place; there are lessons to be learned) ... I would go with Jill. Even though I'm pretty sure I wouldn't like the claustrophobia of Underland any better than she did. I identify with her on many levels and find inspiration in her personal journey of learning to trust Aslan. Further, being able to comfort the Narnians as their world came to such a seemingly hopeless end would be important to me. I would want to be there so I could try to help. So, picking Jill made the most sense to me.

I will say, though, the nice thing about this question is that no matter which one you choose, you still get the ultimate Narnian experience regardless because they all went to Aslan's Country.

Interseting question PhelanVelvel. I probably wouldn't have thought of that. As it is though, if I were to pick a character to have as their great experiances, I'd probably say Peter. Who wouldn't want to be king?

I would take Eustace's. Not only did he get to sail to the end of the world live as a dragon (FLYING!!!!), meet Puddleglum, travel to the north of the world, travel to the bottom of the world, but he also got to see the end of the world, and participate in the battle thereof.

This was almost enough to change my vote, especially the dragon part. But, as I said, almost.

It was a tough choice, but I ended up going with Digory. I think the chance to see the Wood Between The Worlds alone would be worth the experience, but also being able to visit two other worlds (even if one of them was very dismal), and watch one of them be created? Those would be awesome (in the literal sense of the word) experiences.

As for why I chose Digory over Polly... for one thing, as has been pointed out, Digory gets to see another generation experience Narnia as well. Obviously, Polly finds out about it eventually, but it's not quite the same as living it the same way he did. Secondly, I'm taking advantage of hindsight here, but--despite all the frustration and agony associated with his mother's illness, he gets to look Aslan in the eyes and realize that the Great Lion understands his pain. And then, after this, he's given the opportunity to heal his mother. Polly really only experiences those second-hand.

This was a very interesting question, though, and rather difficult to answer--as I said, I almost changed my mind. I think my second choice would be either Eustace--for the reasons DiGs mentioned--or Lucy, for her closeness to Aslan and coming to Narnia 3 times, as well as spending several years there as Queen.