Best TBS outside Nintendo's own offerings:
Civilization Revolution (near perfect transition of Civilization to handheld)
Age of Empires: Mythologies (yes, it's TBS on DS, not RTS, but very well done)
Best economic/building sim with RTS elements (in NA often and imo mistakenly labeled as pure strategy game):
Dawn of Discovery
Best RTS:
Final Fantasy XII: Revenant Wings
Lock's Quest
Robocalypse
Best tower defense:
Ninjatown
Best unknown game on the DS:
Might & Magic: Clash of Heroes
(Like Civilization Revolution this is another example of an excellent transition of a legendary PC TBS franchise to handheld, but with the difference that unlike CivRev - which is a very well done streamlined version of the typical Civ gameplay - M&M:CoH takes a different route and is not the expected Heroes of Might & Magic port to DS; instead it's a unique mixture of strategy/RGP/puzzle elements and one of the most surprising DS games. If you're interested, check out some reviews: http://www.gamerankings.com/ds/960173-might-and-magic-clash-of-heroes/index.html )
