In my opinion, DKC3 GBA has the best graphics, because DKC2 and DKC looked like they just photoshoped an image of the whole level, increased the brightness and pasted the layout in the game =S
And in DKC3, David kind of used a big combination of banks for the midi synthesizer he was using, so he got better results than Jamie's re-arrangements of the original songs in DKC2. DKC wasn't that noticeable, probably because of the instruments used...
Well, that's only the technical stuff XD
I would suggest you go from the beginning, first get DKC, then DKC3, and at the end, DKC2. Why? Because DKC2 got its tone less dark in the port, and DKC and DKC3 weren't changed at all, so by when you get DKC2, you are going to expect a change.
In my point of view... it's interesting to note, that even when DKC3 is the most criticized of all, it's still the best looking and sounding from the ports =P