I used to think that travel insurance was a bit of an unnecessary expense but since having had a nasty accident whilst travelling South Africa breaking both my legs and spending a month in hospital there before being ambulanced and flown home with a nurse, I now realise what an essential expenditure it really is! So glad I don't have a mortgage around my neck trying to pay off all those costs that the insurance company covered me for!
So what is the best insurance to buy, well that depends on what you need cover for? If you are planning on helpxing or wwoofing then you need insurance to cover for a working holiday. Are you flying one-way? Most insurance companies require you to have a return ticket back to your country of permanent residence. Is it possible you might need to extend your insurance whilst still abroad? Will you need to purchase you insurance whilst abroad? Does it cover for adventure sports and activities?
Having investigated different types of insurance the best policy I've found is by a company called World Nomads. They are the only company I've found that covers or allows all of the points mentioned above and they are one of the cheapest too! But don't take my word for it, check them out and don't travel abroad without travel insurance!