Which is the best island to visit on vacations?

United Kingdom/Great Britain, Ireland (both Northern Ireland and Republic of Ireland), Isle of Man, Isle of Wight, Isle of Mull (Scotland), Japan and/or New Zealand.

Either Ireland (Northern or the Republic) or the islands by Scotland.

Isle of Sheppey

The Maldives if you can afford it, but this is really subjective depending on your personality and preferences.

Bora Bora , Bora Bora

Where are you? What do you mean by "best"? Nightlife- Ibiza. Beaches- it's hard to beat the Caribbean since the water in the Gulf is warm and clean. Scenery- the Greek islands. If you like to climb volcanoes- Iceland. 

I agree with Sunshine, what are you into, and what's your budget? Are you on the east or west coast (Assuming you are in the US)?  

Caribbean Islands are cool, but some are third world countries, and so you have to take that into consideration.  Some are very pricey, like Aruba.  Bermuda is fun, but can be chilly depending on the time of year as it isn't as far south as the Caribbean.  Hawaii is really expensive and is a really long flight, but it's an amazing place. 

And then there are places that aren't on most people's radar, like the Azores.  I like the Azores as there are 9 islands, each with it's own flavor.  Most of the people speak English so that's not much of a problem, and it's very affordable.  IT's also only about a 4-5 hour flight from the east coast.  Puerto Rico has a lot to offer, and as it's a territory, you don't need a passport.  And everyone speaks English. Not too bad a flight, either, depending on where you are coming from. 

padre island ..texas takes some beating

I wish to visit Christmas Island someday during Christmas.

Depends what you're looking for in a vacation destination, your budget etc.