The best Shooter IMOP, is Thunderforce II.
Its listed as not working in mame.. however, it appears to be playable now.
Thunderforce II was originally made on the Japanese computer system X68000.
It was later ported to the Sega Genesis / Mega Drive.
The Sega Genesis had an arcade unit that used timers on it.. and TFII was one of the games selectable,
which is why its in mame.
The Sega port is much more limited. Less layers of graphics. Less colors. Less details. Missing a few
game levels... etc. Still, the games plays great, and its a very good port.
Why is TFII the Best?
Many shooters have glaring problems:
1) lose powerups at a high level & your too weak to survive anymore.
2) Too many bullets on screen. Detracts from looks & ruins gameplay
3) Superbombs. Too many = No need to have skill to progress.
4) Unlimited Continues = No need for skill to progress.
5) Restart at beginning of level. Very frustrating, Esp without enough power to fight well..
6) Game is a puzzle. Only one pathway to be able to progress. Pure memorization only = poor gameplay.
TFII solves these issues:
1) Basic powerup level is sufficient for any level, yet game is still very challenging.
2) Only bosses toss huge amount of bullets out. Rest of game is not supersaturated.
3) No Superbombs!
4) Limited number of continues possible.
On easy level, more continues... but game only play till 5th level. Rest are locked.
On hard level, less continues.. yet allows completion of game.
I believe there was also a med level with a similar restriction on levels that were available.
5) No level restarts. If you crash.. the screen keeps scrolling & you re-appear in a moments time.
However, if you are blowing thru too many lives per a single level.. you are never going to be able to
defeat the game.
Many games shut the player down immediately, so they could never experience the mid to higher levels without being an absolute expert. This discourages, and makes the play a lot less fun. In TFII, you can
get pretty far into the game as a beginner.. have a lot of fun.. yet wont get anywhere near the end
until you get good enough to be able to lose less lives per each level.
6) Not much of a puzzle game. Can fly by the seat of your pants, and do well enough.
The main complaint about TFII was the Top view levels. They are a little clunky in control... and also,
most people didnt realize that the weapons in top view shots both forwards, and drops bombs on the
ground targets. However, the bombs are close range only... so, even tho it looked like you were shooting
the ground turrets... your bombs were too far away to reach them. The top view is also a bit of a maze, and
you can easily get lost in them.
However, the Horizontal sections are some of the most awesome levels Ive ever played.
You can skip many of the beginner levels by pressing "A+B+C+Start" at the Title screen, to accress
the hidden menu options.
Listen to the music sections as well. They are some of the best tunes Ive heard in a shooter. Very
futuristic and alien sounding.
Sadly, TF3 and up dont compare to TF2. TF3 took me a day to beat... where as TFII took me like
a year of off and on play... and was much more enjoyable. The graphics on 3 were mostly too low res and
blandly repeated. The music quality was lower (highly compressed & or lower sample speed), and
a lot less inspiring. The powerups system was ruined... as they made your bullets too large, and spread
out all over, littering the screen. TF4 was too hard.. and equally uninspiring, much like 3.
TF3 and up tried to use more organic look to them.. however, because of the Genesis's very limited color
palette... organic stuff looked horrible.
Ive beaten the game starting from level 5. One day Ill master the top view sections, and try
to beat the entire game from level 1. I also have to try to beat the much harder X68000 version,
now that Ive finally got hold of it

[youtube]http://www.youtube.com/watch?v=_tdii-L1zTw&feature=related[/youtube]