Pretty much all Battle Network titles, 3/5/6 in particular just don't age (throw MM Zero 2/3/4 into the mix as well).
When there's something so unique that nothing comes close to it and there isn't something newer that replaces it + a pristine art direction, people will revisit it forever and that's something these games in particular do.
Anyways, Summon Night Swordcraft Story is really neat and it holds up as well. The Boktai titles, even without their sunlight gimmick are awesome for bite-sized Metal Gear style stealth.
Gunstar Super Heroes and that one Astro Boy also fit since Treasure are the kings of insane action, their stuff lives beyond time itself.