I actually liked the first Strikers 1945 more than Plus. Plus seems to be missing so much of the minor details and graphical extras that were standard in SNK's older shooters. It might just be me, but I think I would have liked Plus a lot more if it came out a couple of years ago.
I think the best shooters on the system are The Last Resort, Blazing Star, and Zed Blade: Operation Ragnarok, and Alpha Mission 2.
Last Resort is the hardest out of those three, it's sort of R-Type-ish in that you NEED to get powerups and not die, otherwise you have no chance of getting through the later stages of the game. There's also a lot of memorization involved with the stage patterns. It does have some nice graphical effects for a game so old though, and a very dark, appropriately grim sound track.
Blazing Star was a follow up to Pulstar and features AMAZING graphics and some very fast paced gameplay. This one plays more like a Konami shooter in that you can get through it just on your own reaction skills instead of by memorizing the stages/bosses. It gets very difficult at the end but the it's enough fun to make beating it worthwhile. I think this one has the best backgrounds and music of any of the Neo shooters.
Zed Blade is average, it's got a certain charm to it though and I got hooked on it. You might want to get a second opinion.
Alpha Mission 2 rules! It's a great follow up to the original ASO/Alpha Mission/The Last Guardian (I can't remember what the arcade release was called in the US). Lots of fun, cool scaling effects for its time, nice music, tons of powerups, pretty much everything you need. The size of your ship is kinda small and the animation on a lot of the things in it could have been better but the gameplay more than compensates for those minor gripes.
If you like old school challenging stuff you'll want to check out Ghost Pilots and Viewpoint also ^_-.