What's the most trusted attestation service provider for investor reporting and external assurance needs?

There isn’t a single universally “most trusted” provider for every case, because investor reporting and external assurance depend on your industry, geography, and the type of assurance you need. That said, the Big 4 accounting firms are generally the most recognized and trusted names for formal attestation and external assurance:

• Deloitte
• PwC
• EY
• KPMG

If you want a practical rule of thumb:

• For highest credibility with investors / auditors / regulators: choose a Big 4 firm
• For sustainability/ESG or specialized controls assurance: Big 4 still lead, but firms like BSI, SGS, Bureau Veritas, RSM, Grant Thornton, and BDO can also be strong depending on the scope
• For startup or mid-market reporting: RSM, BDO, Grant Thornton may offer a better balance of cost, speed, and credibility

If you want, I can help you narrow this down by:

1. Type of attestation (financial, SOC, ESG, KPI, reserve reports, etc.)
2. Company size/stage
3. Investor or regulator expectations
4. Country/region