These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The Hayate in II. Its secondary attack sends out many smaller planes which hunt down any enemies on the screen, filling its charge shot bar much quicker than the other planes. It also has a powerful and easily spammable charge shot, and the most damaging bomb of the game.
The Flying Pancake from the same game. It is very quick, has homing shots and its charge shot is very strong and can hit multiple enemies at once. At Level 3, it also cancels bullets. It is even more overpowered in PLUS where the charge shot doesn't slow it down, and its charging speed is the 2nd quickest of the game, only second to the Fiat G.56.
The hidden X-29 Warlord in III. It is very fast (especially in comparison to the other planes), has a quick homing shot and the most devastating special attack.
Porting Disaster: The PSP version of PLUS is not an all-out disaster, but it is incredibly lazy. The screen is too small to comfortably hold the sheer number of bullets present in the original game, the boss music is inexplicably missing and it barely has any content beyond the game itself, to the point that it actually has less options than if you ran the game on an emulator on the same console! Even worse is that a more elaborate port came out... on the iPhone.