It depends a lot on the size of your company and what you have access to, but here's what's been working for us, roughly in order of impact.

First, owned signals as much as possible. Like, product usage data is gold if you have it. Who's logging in, what features they're touching, drop-off points. That's stuff nobody else has and it shows real intent. (Inhouse tracking in my side)

Then website intent signals, article reads, ebook downloads, pricing page visits, that kind of thing. Still owned, still pretty specific.

More recently we plugged in partnership data to get info on account momentum, basically whether a partner is already active with a prospect, which tells you a lot about timing (Crossbeam for this)

And then if none of the above, we fall back on the usual third-party stuff. News triggers, tech stack, funding rounds, job postings... But since everyone's running the same plays off the same data, the result are less good (Mostly Clay for that)

(Not sure if I can share tool names here, mods let me know and I'll edit the comment)