Well
that is definetely a matter of personal preference. Discussing videogaming preferences is almost like discussing religion.

For instance - I started playing videogames in the early eighties - one of my preferred games on the arcade (at that time) was Rally X. I was awestruck with the excellent graphics of Xevious. Truxton was excellent!
Later I had an Atari 2600 - I loved Keystone Kapers, Pitfall, Pressure Cooker, Megamania... the graphics were basic to say the least. But gameplay was absolutely addictive.
I totally skipped the NES and SMS console (nothing against those - my parents just did not have the money by then). At that time I was fiddling around with the ZX Spectrum... which was not a beast for gaming but had some good conversions. I also played a while with some friend's MSX - the Konami games for that system were a blast and Zanac was a total gem.
Back to the arcades I got hooked again on some NeoGeo games such as Aero Fighters 2 and Metal Slug.
I am impressed with some of the arcade conversions on the NES, like Bubble Bobble, Burger Time and Son Son. Sure the graphics were simple (and the arcade graphics sure were) but the gameplay is king.
The SMS has some decent titles - nice graphics and music. I find Sonic a little too convoluted for my tastes - but definetely there are plenty of Sonic fans that might flame me for this one.
Bottom line: keep your options opened, concentrate on gameplay and you will find excellent games on all systems. Atari 2600, Atari 800, MSX, Spectrum, SMS, NES, Atari ST, PC Engine... they
all have excellent games that will sure fit your style.
And they are all being emulated on the GP32. What a dream come true.