That's the thing... I don't know what terrain to use to make it look good in all 3 climates.
If you don't intent to use blizzard cliffs (which you shouldn't anyway), then I recommend taking a tileset with two cliff tiles that look good on their own (when not being used as cliffs), as you can not replace cliff tiles later.
If you want to have all climates in your map, I recommend going with the northrend or lordaeron winter tileset. Both have acceptable cliff tiles that look good when used standalone.
Then fill it up with the tiles you need for the other climates. You can check different tutorials to find out the most popular tiles.
I recommend taking at least 3 different earth tiles:
One for the winter terrain (lordaeron winter or frozen throne), one for the 'green' area (I recommend cityscape, all others are too colorful), and one for the desert (barrens).
Then add more utility tiles: grass tiles (ashenvale grassy dirt is very popular), possibly one or more tiles from barrens, if you need, some from cityscape/village (you need cobblestones for villages), also, you need some stone tiles - as you want both desert and winter in your map, its gonna be difficult here. You should almost have all tile slots used now.
If you really need so many different tiles in your map, I really recommend you to learn how to use Multitiles. Check for example the multitile in the skins section and edit it the way you like it.