Wanting to play this series, what game should I start with?
Metal Slug
The first one
Just play them in order, they're all great games.
Metal Slug 5 and go backwards from there.
In order. Skip 2 and play X instead, it's basically a reskin with improved performance, powerup placement and a pumped up difficulty
Fio a bes
Official Metal Slug Waifu ranking
Tarma > Fio > Marco >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> Eri >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> everything else
This.
Get the PC collection and you're golden.
MS2 is a decent place to start, exactly because of the lower difficulty. The second level is a good place for a newbie to learn grenade techs.
All of them are good games and there aren't many gameplay changes between each game, so you can start wherever you like.
Fio best girl, if you think otherwise you're wrong and/or dumb