I would definitely side in favor of a Goldsource mod.
I would recommend looking into quality multiplayer mods, like Science and Industry, or maybe Bleach: Zanpakutou Senshi, that don't necessarily get a lot of attention. You'd probably need to get people to play on your server, asking them here is a good place to start. There's a list of mods on moddb:
https://www.moddb.com/mods?filter=t&kw=&released=1&genre=&theme=&players=2&timeframe=&game=1&sort=visitstotal-descYou could also change the mod every once in a while, see which ones "performs the best", so to speak.
I think Sven Co-op is doing okay in terms of servers, especially since it supports p2p. However, it would be great to have a server running maps from the TWHL community. It would give more incentive to mappers for individual projects and even competitions. And we could all play on it. By the way, did I mention I made a map for Sven Co-op ? 😇