I'm gonna step up and defend Slug 5.
First off, the music is the best of the series. Sampled instruments and really catchy tunes get me into the game, and I often wish there was a soundtrack available.
Then we have the enemies. I don't know, for me, its a nice change of pace to fight a new army of bad guys. They look like Snake Eyes for some reason, and that's cool.
Although sadly, this game is not what it could have been. Yes, the secret bosses would have been hella cool, and there could have been more alternate paths. However, its still a damn fun game, not incredibly difficult, but not a cakewalk either. Definitely a game that I will play over and over just for the sheer fun of Metal Slug.
However, my all time favorite is still Metal Slug 3. To me, its the ultimate test of short attention span hardcore gaming - do I have the attention to sit through the entire game? Most of the time.
But I still hold a soft spot for Slug 5. Last Slug on the Neo, yes. Could have been better, absolutely. But still, a worthy addition to the series.
Pick it up now, or you might just regret it later when the neostore is sold out.