From the games I own-
I'd say
UNDERRATED:
Image Fight
Everytime I play this it screams "classic" to me. Yeah- it's hard. But I can't beat R-type either.
Image Fight's gameplay is truly classic! I totally love the difficulty level of this game... it feels so perfectly balanced. Usually when I get killed it's because of a minor mistake, so I always feel that I could do better.
The game really keeps you on your toes, and the red pods add such a great feel to the gameplay it's almost indescribable.
If ever there was a shooter that really put the players concentration, reflexes, and memorization/execution skills to the test, without including unneccessary eye/ear candy to distract the player's attention away from the otherwise bland gameplay found in so many other shooters.... Image Fight is it.
It's all about the overall experience.... Image Fight's graphics, music, and sound are to me solid, but unspectacular. I feel the gameplay, however, is spectacular.