Can anyone nominate any games out there that are really good when it comes to how moddable they are?

Jagged Alliance 2. Access to the source code does wonders for modding ability, though the engine does have quite a few limitations.

Call to power II

Star Wars: Empire at war (mod like stargate out there i.e.)

Space Empires IV: tons of mods out there

the tes games

morrowind
oblivion
and skyrim

some would say that its the mdoders that make these games great

fallout 3 and NV are also quite mod friendly

sims 2 is also easy to mod

I think we need a complete GOG mod database .

Or you could just look in there:

http://www.moddb.com

me thinks the biggest collection of mods available ;)

Neverwinter Nights, both 1 and 2

With both you can create whole worlds and adventures

Infinity Engine games: Baldur's Gate, Icewind Dale and PST.

http://weidu.org/

Unreal Tournament 2004. Even comes with a complete set of modding tools (or at least a whole suite is freely available).

Mount and Blade

Civilization 4 is highly moddable. Many of the civilization series games are, to one degree or another.

Dwarf Fortress is highly moddable but it's a hard game to learn.