I'm tempted to say HHB, mostly because it's my favorite book and, other than TLB, it's the one that the most takes place in the Narnian world. Buuut I can't stand heat and a trip through the desert would not end well for me.

Would it be cheating to say TLB just so I can go to True Narnia at the end? Of course the battle itself is the most harrowing thing in any of the books and it would be so painful to have to watch Narnia die, but... every time I meet up with NarniaWebbers in real life and then we have to leave again, I wish a little more for going to a True NarniaWeb where we can always spend time together in person, and with our families and non-Narnia friends around, with no fear of a job to go back to. Like in the book itself, I think the reward might be worth it.

Otherwise, I'd probably say The Magician's Nephew. You get to see three different worlds other than ours, if you include the Wood Between The Worlds, and two of them would be so beautiful to see.

I also think it would have been fascinating to see Strawberry turn into Fledge and that whole scene. And then experience flying across a brand new country on a horse. This would be brilliant.

Lucy in The Lion, the Witch and the Wardrobe, after they have been crowned kings and queens. Getting to know the land and the inhabitants of Narnia for years would be pretty thrilling.
In second place would be Polly in The Magician's Nephew when Narnia is created.

I would love to go through Aravis' story line. It really is one of my favorite. Especially the part where she had to try and sneak out of the Tisroc's palace. It's one of my favorite parts with her in it in the book.

I think this changes everytime I read the series. Right now, I would really love watching Jadis in her life. Starting from childhood, the start of her rival with her sister. Seeing how they grew up in Charn and watching the fall of all that up to the point where the bell is finally rung. Her side of the story, told from her view point. I think that would be pretty fascinating.

Wow.. this is hard but I have 3 scenes that I would love to be in. 

1. Pevensies Coronation/Golden age

Ever since I have read the series and finished it, I have been dying to go to Narnia but most of all to be at the Pevensies Coronation. Just seeing them after all they had been through, finally being rewarded with the titles of King and Queens. (and High King. Don't worry I didn't forget you, Peter ) I would love to be there and meet the Pevensies in real life and talk to them.  But most especially see Aslan since during the whole life of Narnia, he appears rarely to the creatures.  I would especially like to see the Pevensies rule during the Golden age because it would be a prosperous and peaceful time in Narnia. 

2. The Creation of Narnia 

The Lion's song is what intrigues me the most. I would love to hear the lovely  song that Aslan sings and see how beautiful and young Narnia is when it is born. I would like to see the animals talk and the silver apple tree. Though Jadis is there, nothing could be more rewarding then seeing Narnia's birth. 

3.  Aslan's Country (Last Battle) 

Though this is the most depressing book in the whole series, like people mentioned before, the reward is greater than all the sadness. I would like to feel  the peace and happiness when I go to Aslan's country and see all the good Narnians in the series. I love the feeling that Aslan is there to protect you and nothing bad would ever happen. There would be joy and adventures surrounding you every single day. 

The Magician's Nephew. Four worlds to experience: Edwardian London, Charn, The Wood Between the World, and the Dawn of Narnia. If I had to pick one scene, the journey on Fledge's back to the Garden of the Silver Apples. I would love to sleep next to a horse and be covered by his wing.

Runner-up: The scene in The Silver Chair where they walk over the Giant's Bridge that crosses the gorge. I have no fear of heights, obviously.

Hmmm... well, Lucy is the character I've always identified most strongly with, so if I could swap places with her in any of the books she appears in, I'd be happy. (Except I don't know that I'd really like to go voyaging on the Dawn Treader, exciting adventures and wondrous new discoveries notwithstanding... I hate the thought of no running water and no proper way of washing my hair for so many weeks on end. But it's made clear in the book that Lucy loves seafaring and had previously done a lot of it during her years as a Queen in Narnia, so if I were her, I presumably wouldn't find it so bad.)

Seriously, though, a whole lot of my favourite scenes are Lucy's experiences — stepping through the wardrobe for the first time, romping with Aslan after his resurrection, seeing the statues brought to life; then in Prince Caspian, where she dances with the waking trees and finds Aslan again in the clearing — and her encounter with the Magician's Book in Dawn Treader (which of course has some very disconcerting parts, but I would love to read "that" story that she could never remember afterwards!), and the journey through the silver sea of lilies to the Utter East...

Two other stand-out episodes that I would love to witness for the scene itself, regardless of which character I might be: the creation of Narnia in the beginning and the discovery of the "real Narnia" in Aslan's country at the end. Probably THE two most wonderful (outright breathtaking) lengthy scenes in the whole series, with the way Lewis describes them and builds them up — absolutely unforgettable.

I'm not sure I would like to be in The Magician's Nephew for the entire story — I think I'd find Charn and Jadis too scary in real life. (Though flying on Fledge would be amazing.) And the drawback of being in the ending of The Last Battle would be that I'd have to be dead — in this world, at least — and I'd prefer not to do that just yet, if I can help it. But it certainly is the best and most convincing description of heaven I've ever read.