besides already recommended Fringe, Millennium and Halt and catch fire I enjoyed at same time as X Files in 90s also new Outer Limits, opening credits were great and episodes are always thought challenging, I'm just rewatching it and still enjoy it, though technical quality is horrible since I think FHD was never released

but if you wanna straight up X Files just based on 50s check out Project Blue Book, it's like retro X Files, you won't get closer than that to X Files