Best for the job!!
 

Injection or carbs?
Ally or spoked wheels?
Chain or shaft?
Canvas or metal panniers?
1,2,3 or four cylinders?
Lightweight or heavier bike?
Ideal cc?
Japanese or other?.
At least 95% a or b road. 5% or less other

In my humble opinion the best bike for the job is the one that you will most enjoy making the journey on, if you have a bike you particularly like then take it, what ever it is.

Depends on the job and how much cash you have, but in general:

Japanese
Middleweight, 650-800
1-2 cylinders
Fuel injected
Chain drive
Tubeless tires, spoke, but for that amount of road, cast wheels are fine
Canvass/soft bags
19"+ front wheel

Sounds remarkably like a 650 V-strom to me, or XT660Z (less the tubeless tires). BMW F650/700/800GS also fit the bill.

Tubeless wheels can be had from Alpina (for a princely sum) for bikes that come standard with tubed, spoked rims.

I would go V-strom 650, Tiger 800, F650/700/800GS if I was buying on those criteria - take 'em all out and see which one speaks to you.

Just my opinion and worth exactly what you paid for it.

Nice one Dosh.
I am not so kean on bid bore singles and twins. Saying that i have had two teneres, one ttr600 , two firestorms, an rsv mille.
Don't get me wrong if you want it more sports based they are great but at slower speeds with the power pulses they don't like low revs. Some of the cruiser based engines with low power are much better. With the singles you do get good mpg.
Forgot another! Belt drive. Personally this sits in the middle. Never tried one but i eould like to, no mess, light, quiet. Sort of best of both worlds.
Why tubeless tyres?

"Why tubeless tyres?"

Much easier and quicker to repair punctures.

Suggest you avoid belt drive if going off road - a rock in the belt system really upsets things.