Ahh yes, for Ben/Leslie fics (it's... the only Parks fic I read, hee) I would definitely peruse the recs list mollivanders posted. I particularly recommend anything by cypanache, especially "Snapshots Left on the Negative" and "A Beautiful Mess". I also like "A Long, Long Journey to Capital City". I was also recced this ensemble piece, but I haven't read it yet (it's also... apocalyptic zombie fic so I don't think it would fit your qualification of feeling like the show).

As for The X-Files, I wish I had a Delicious account a million years ago when I first started reading fic for the fandom. I do recommend anything by Prufrock's Love, and Fugues in the Key of X has a pretty incredible recs list.