You really can't look at most Neo Geo games, Metal Slugs definitely included, as something to "beat" and be done with. They are arcade games designed for a fairly quick fix, and they are totally about the replayability to me. Since the Slugs aren't competitive 1-on-1 games (which you do find on Neo in the fighters and stuff like Magical Drop), it really comes down to whether you enjoy firing them up and either:
a) doing a full run through of the game, credit feeding to your heart's desire. Maybe you're trying to do it in as few credits as possible and improving on your best number, maybe you just wanna see the scenery or goof around co-op with a friend, whatever.
b) trying for score, though I don't particularly find run-and-gun platformers hold up as well in that regard as something like a typical scrolling shooter with some intricate scoring mechanics (think Cave games)
c) learning the game enough to where you can do a 1 credit clear, to truly claim you can "beat" the game. Or, for a slightly more lenient variation, give yourself 4 credits like the standard Neo Geo home cart setup. I remember the U.S. version of the Xbox port of Metal Slug 3 limited you to 4 credits, and learning the massive final mission enough to finish it with the limited credits really did feel like an accomplishment. I could always get to that mission on my first credit, but oh boy does the last gigantic mission turn up the difficulty.
That said, Metal Slug 3 is by far the best of the series in variety since there are LOTS of branching paths - generally at least two majorly different branches in each level. I kinda feel this is why MS3 has always been so popular, especially with people who are more casually into Neo Geo games. Me, I kinda prefer the completely linear and short MS1. I could play it 100 more times and still find it fun just to go through it again, similar to how I feel about stuff like Streets of Rage 2, Sonic 1, Castlevania... But I understand where some people might be uninterested in that and find MS3 provides them a lot more value.
Yeah, VC Baseball Stars 2 is actually one of my most played Wii titles. It also appeared on the PS2/Wii/PSP SNK Arcade Classics Vol. 1 compilation.
Wouldn't surprise me at all if we see it before long, though I'm personally a lot more interested in Super Baseball 2020 because that hasn't ever had a Neo Geo accurate port.
https://www.nintendo.co.jp/software/feature/akeaka.html
Top 20 Neo Geo titles on Switch (in Japan?) from March to July:
Note: There are currently 26 Neo Geo titles available on Switch.
Since the two most recent releases don't appear (Super Sidekicks this week; Samurai Shodown last week, which I would expect probably did pretty well), seems likely that this list covered the first 24 releases in the series at the time it was compiled. That means the four games that didn't make the top 20 are:
Over Top
Galaxy Fight
Sengoku
Fatal Fury 2 (I guess people knew FF Special was coming 3 weeks later)
Aside from shared surprise that Garou was so low (maybe due to more ports, including the recent and quite good PS4/Vita release from 2016), it pains me a bit to see Neo Turf Masters only clocking in at number 18. That has also been re-released quite a bit, but it still deserves to be #1 forever.