there really isn't one. each game is kinda it's own thing.

reloaded is the original game, and all the standalone games that were built off it in one, so there is a lot in there.

Labyrinth has this whole virtual thing going on, but is more like what if instead of you and a partner you took a army. It doesn't play a lot like the others.

Foresite (I just started playing it.) is a lot like reloaded but it's got some kinda character level buff system you can invest points into, so that kinda changes the formula a bit.

Overall I would say pick whatever sounds good to you.
Labyrinth can be a bit easier at times since you can let the AI basically carpet bomb a dungeon if you leveled them up enough.

There's no story but there's some references you would get In Reloaded if you play Lotus Labyrinth before it. It's a pretty insignificant reference since the stories in these games are all pretty nonsensical to begin with but with more context of a certain boss in Reloaded you may get a bit of a laugh out of it knowing the story of Lotus Labyrinth. Lotus Labyrinth is a remake of an older game so they added some references in Reloaded but it's not really that worth playing Lotus Labyrinth first.

It's probably better if you just play the release order of Reloaded - Lotus Labyrinth - Foresight since the newer games add quite a bit of QoL. Lotus Labyrinth also adds a lot of new characters to the roster that are not present in Reloaded due to it being an older title. Foresight however is a mixed bag entirely and I would honestly stay away from the game until there's a sale.