For the most part they are self-contained, and could theoretically be played in any order, though there are a few references here and there. I guess the best playing order would be to start with 4, 0 then 5 on a PS2 emulator(some will argue it's better to play 5 before 0, and that is fine as well, but I think the story works better if you play 0 first, but this is just my opinion). If you are curios to see hoe the older games handled then play 2 on either the PSX or the 3DS emulator, then 3 but make sure to pick the jap version, since the American version cut all branching storylines from the game, and it recently got a transaltion patch so there is no excuse. If you have a 360 or Xbone, pick 6, it's really cheesy but the combat is fine. Even if you aren't curios to see how the older games played or have an xboxe for 6, I think it's best to play 7 after finishing 4,5 and 0 as it makes the most references to those three games. There are 2 PSP games, that I haven't played yet, I heard they were decent. There's also a Multiplayer only game called Infinity on PS3 that I haven't played, so can't comment, though 7 does have multiplayer as well. Stay away from Assault Horrizon on 360, PS3 and PC(not to be confused with the 3DS game which is actually a remake of 2 and it is actually good) as it is a piece of shit and nobody in the community likes it. If you still want more, then play Project Wingman. Hope this helps anon.
>tl;dr play 4,0,5,2(original or 3DS remake),3(jap version with translation patch),6(only on 360 and xbone),7, project wingman