I've used a lot of routers and my favorite is still page.js[1]. It hasn't been updated in years. But it's small, is Express-compatible (i.e. server/client routes can use the same code), and, more importantly, is hackable. I'll never use a router tied to a certain framework again (react, nextjs, etc.) because you trade flexibility for perceived convenience (e.g. using folder structure as route structure, or React component tree as route structure). But it's a terrible trade-off that paints you into a corner later, IMO. Routing can get really niche and site-dependent, so having it fully under your control is worth it.

[1] https://github.com/visionmedia/page.js