Hi Poppy
Wow - it's hard to choose from those three as they all have so many things going for them.
For sunshine, you can't beat Australia. Travel during the British winter for the hottest temperatures, or during our summer months if you prefer it cooler. You'll also see see some amazing natural sights, such as Uluru (Ayers Rock) and the Great Barrier Reef.
For culture and spirituality, I would go for Argentina & Brazil. You will find a very different, vibrant way of life in latin America (think of the tango and the beach life of Rio) and so many legacies of Spanish colonial rule in the cities. The Catholic church is still a big part of life there, so you would see much in the way of spirituality.
For a new and exciting experience, Route 66 would be a great choice. Just You has only recently added this holiday, so you would be one of the first to try it. If you are interested in American culture and music, I think this one would be unforgettable.
I hope this has helped you rather than made the choice harder!
Kind regards
Olly, your editor