Metal Slug wise I still prefer 1 to the entire rest of the series. It just feels so well balanced - tough but its believeable that I could actually complete it legit if I practiced just a little more (rather than credit feed).
2 is nice, but I prefer the music and lack of slowdown that X gives you. But X looks washed out colour wise compared to 2. That said, I love the game and like 1 I'm not too bad at it.
3 I love, but fuck that final level. It completely destroys the difficulty curve for me, both because the difficulty ramps up quite a bit and because its probably as long as the first 4 stages put together. That said, amazing soundtrack and I loved all the different paths in the game. Its a game I love to replay, but that final level means I'll never seriously try and clear the game's final level since I don't have that much patience!
4 I don't like. Feels souless and I hated the new characters (especially Trevor I think his name was?). Final level/boss was a let down as well after how epic 2/X and 3 were. Only good points on the game for me were the soundtrack and the dual heavy machine guns.
5 is ok. Not the greatest game in the series by any stretch but its playable enough. I find the game more interesting to read about than to play, since there was so much cut content in the game, I wonder what the game would have been like if the game was given more time.
6 is the only game I've not finished, mainly because I just didn't give it enough time. Not sure why there is this weird Vaseline filter on it making everything look so blurry, its also bastard hard even by the series standard, the game kicks my ass from level 2 and up. Usually I can clear the first 3-4 levels with few problems!
7/XX I really like. I've mainly played the Xbox 360 port but its on DS and PSP as well. Again, like 5 its not a groundbreaking title or as good as older games in the series but its good solid fun, netcode seemed decent as well since I'd co-op the game with a friend quite a bit.
As for spin offs:
Metal Slug Advance (GBA) - Average at best. Respawning enemies and run of the mill levels/bosses lessened my enjoyment of the game. Track down if you see it cheap since its still a fun time waster.
Metal Slug 1st/2nd Missions (Neo Geo Pocket Colour). Like other SNK franchises on their short lived handheld they managed to create two excellent handheld games, that look and play great. Unlike Advance the games feel more suited to handhelds and are just a joy to play. I really recommend tracking these down if you can find them.
Metal Slug 3D (PS2). Not played much of this one, its MS but in 3D and as you can expect its not really that great. It comes from that same time period where many games were being forced into 3D for the sake of it, so the camera isn't great and neither is the lock on aiming. I still kind of enjoyed the game, but the fast paced action and awesome sprite animations are replaced with a rough 3D action game with bad polygon graphics.
S and New Island are terrible
