Finding all of them legit might be hard cause there's some weird fucking spin off and some might mobile game not accessible anymore. I'd stick to 1-2-3 and birth by sleep. Word of caution, if you can't understand the title, don't bother.
Actually playing 358/2 Days and Coded is difficult, but the rest of them are totally playable through the HD collections, which at least recap the stories of the two I mentioned (as well as the mobile MMO in the third collection, 2.8). If you want to have any grasp at all of the wider story, I would strongly recommend
not just playing the numbered entries, and at the very least I think that Chain of Memories is worth playing on its own. The deck-building system works better than it sounds, mostly, and the story is genuinely one of the best in the series.
And speaking of recommendations, Days is put before 2 in the timeline and in the HD collection boxes, but should be played/watched
after 2, for two reasons: 2 came out before Days, and as a result there's a lot of stuff in 2 that was intended to be seen before Days despite taking place chronologically after.
Or I guess just don't give a crap, but even in that case you should at least try all of them that you can get instead of just playing the numbered ones.