I have a couple of games that I want for the DS : FFIII , NSMB , Metroid PH , MKDS , Resident Evil DS , Zelda PH , Battles of POP ... and well , I really cant think of more ... maybe I would buy Rayman DS ( personal favorite )

FF3: only buy if you like old school JRPGs like FF1 and FF2 (FF origin on the PS1)

Metroid PH: Good multiplayer, weak/boring single player

Resident Evil DS: haven't played but I would give it a pass, better play the original or the GC remake.

Battle of POP: I have heard both bad and good things from it, bad probably because of the expectations of the POP name and good from those that went with an open mind and thought "actually it's not that bad" so I would suggest trying it first.

But what I would say is: STAY AWAY FROM RAYMAN DS!!! It is Rayman 2  which should be a good thing but it is a shovelware port from the beginning of the DS's life (a bit like what we got on the Wii this past year) and the broken controls totally ruin it. Better stick with the original or Rayman Revolution.

So if I would buy one tomorrow , I would want to know what other games I could pick up . I like

platformer: NSMB, Sonic Rush (and SR Adventure probably but haven't played yet), Super Princess Peach (not the best but fun so buy second hand/bargain bin), Megaman ZX and Kirby Canvas Curse come to mind.

action/adventures: Zelda:PH... I'm sure there are others but I don't think I have played them.

action/RPGs: Castlevania:DS and POR too probably but I haven't played it yet. There is also some Roguelike games like Izuna and Etrian Odyssey that would fall in this category if you like getting your ass kicked hard (again haven't played them).

tactical RPGs: I don't know if Final Fantasy Tactics:Advance 2 is out yet but the first one was good on the GBA (though it disappointed many FFT fans).

shooters: Beside MP:H I don't really know. Megaman Zx if you consider it more shooter than platformer (I don't hence it being in platformers earlier in the post).

racers: Outside of Mario Kart DS I would stick with the PSP for that as that is a genre in which the PSP's bigger screen and more powerful hardware offers more than two screens (though a map screen is nice) with a touch screen.
 

PS : please , no stuff , like : Whats the matter , the PSP doesnt has enough greta games no more ? Except if its a joke of course :P

Oh, almost forgot: What's the matter? the PSP doesn't have enough great games anymore?

 If you do decide to buy a DS I hope that you will enjoy enough games on it to make the purchase worthwhile.