Fiji, Samoa, Tonga or Cook Islands

Hi, I am currently in New Zealand for a language stay. I will finish on 15th of March and then I have 3 weeks off before I go home. I would really like to visit some of the other Pacific Islands. In this 3 weeks, i would like to see 2 or mabe 3 Islands (not more). I want to have the real Island experience (backpacker) but in my last week I want to relax in a Hotel or something. I have never been to one of the Pacific Islands yet, what Island would you recommend me? Fiji, Samoa, Tonga or the cook Islands? Or another one?
I'm looking for:
- a cheap but good Place to stay on my first 2 weeks. What Island is the cheapest?
- Which Island has the best weather during this time? (15. March - 5 April)
-The most beautiful scenery

Thanks for your help in advance! (Sorry if my english is not that perfect :) )

I would probably go to either Fiji or Samoa. Both are really good value.Then it depends on what you want to do, what type of accommodation and what you expect.
Fiji is more backpacker orientated so easier to get around, sort what you want to do, plenty of dorms at a reasonable price and a chance to meet other travellers to do stuff with.
As a single traveller who likes my own space ( and lives on a budget) I like the beach fales in Samoa .But the information is harder to sort and the backpacker network not very established. Maybe just go for fiji and check a few different islands around fiji.
If you are interested in trying out budget places in Samoa have a look at my facebook page Aba Samoa, from my last trip in June/July and contact me if you have any questions