As owner of quite some Neo Geo Pocket games I have the feeling the rarity-ratings are not real right. Some games are put much to high in rarity while others are much more common. So I give my opinion about what it should be in my humble opinion for every game/ items which is misrated.

Baseball Stars JP R3 EUR R8
Japanese version is quite easy to find, the English version is really a pain to find (it took me 2 years to find a copy)

Dive Alert (both versions) JP R3, USA R5, EUR R6-7
I think this game is rarer as most people think, for Europe the rating for Matt's version is completly wrong as they are both hard to find.

Dynamite Slugger JP R4 EUR R6
European version is much rarer then the Japanese. The Japanese version isn't so rare but need some search to be found.

Evolution JP R4 EUR R7
Evolution shows up on ebay still quite sometimes, It is one of the rarer games but I think a R7 rating is more reasonable as the game isn't that hard to get if you have the money. Japanese version isn't hard to find, only don't pop-up so much as the game is complete in Japanese

Faselei JP R4 EUR R6
The Euro version isn't as rare as much people think, it's mainly the reputation of the game which kes it highly sought. So I think a R6 is more reasonable.

Gals Fighter EUR R7
I think the Euro rating is a bit too low and should be 7.

Magical Drop EUR R6
Magical drop in the European version is hard to find these days and is for sure much rare then the R4 it's mentioned now.

Mezase! Kanji Ou JP R8
This game is really hard to find complete boxed and is a R8 for sure

Mizuki Shigeru no Yokai Shashinkan JP R8
I think R8 is more reasonable as the game is also a hell to find.

Neo Geo Cup '98 Plus EUR R8
Much harder to find then most people think. It's one of the games the two games I still miss in my NGP-collection.

Neo Poke Pro Yakyuu JP R6
Japanese version sold much more copies then most people realise, cmore a R6 then R8

Pachinko Hissyou Guide Pocket Parlor JP R9
Almost impossible game to get, maybe the rarest game on the NGP.

Pocket Reversi EUR R8
Less rare as people original thought, but still the hardest to find English Color release

Pocket Tennis EUR R8
Damn this is the other game I still missing, damn hard to catch this game in English version.

Puzzle Link EUR R7
The first part is much harder to find as part 2 so R7 is much more reasonable.

Puzzle Link 2 EUR R6
Think it's a bit harder to find this game so maybe a 6

Samurai Shodown EUR R7-R8
Very hard to find this game, cost me a damn year to find a copy.

Samurai Spirits Special Box JP R5
Not that rare combination of NGP with SS, shows up once every while.

Sonic the Hedgehog Pocket Adventure EUR R3
Certainly not the most common game, still sold a lot but more R3.

This are my suggestions, so I maybe you can give a reaction.