I think it depends a lot on where you're bidding, but I'd probably use a mix rather than rely on one source.

For Europe, TED is the obvious starting point for official notices. SAM.gov is essential if you're looking at US federal contracts. Contracts Finder is useful for UK opportunities, and Tenderwell is interesting if you want one place to search across multiple markets instead of checking each procurement portal separately.

There are also bigger commercial databases like GovWin for teams that need deeper US market intelligence. The main thing I'd look for isn't just "how many tenders does this tool have?" but how good the filtering is. If I'm still opening 50 irrelevant PDFs to find 3 opportunities, the aggregation hasn't really solved the problem.