I’d really like it to be nixd, but it currently lacks code actions, which ends up being most of what I use the LSP for in nix land.
The completion features are nice in theory, but take configuration effort and aren’t great performance wise. I find nil is a more featureful zero-setup option, and would therefore be my preference for a default setting, with nixd hopefully replacing it in the long term.
But that’s just my 2c.
FWIW, I’ve had that issue open for months intending to sit down and just implement my most used actions, but I hate doing C(++) development so something else always takes priority.