I used to play Frostcrag Reborn.
Then started using This Mortal Crag
Then found out there is Frostcrag Reborn de-isolated.

Am bit confused.
Which one would you advise to use?

Thanks.

Haven't played "This Mortal Crag" so I can't comment on that one. Between the original Frostcrag Reborn and Frostcrag Reborn Deisolated I'd recommend Deisolated. It's essentially the same mod, only remade in a way that offers more compatibility with other mods. For example, you can run Arthmoor's Frostcrag Village with Frostcrag Reborn Deisolated using an included patch, but the older Frostcrag Reborn isn't compatible due to FormID issues.