Short answer — for most people the easiest Demat accounts to open online in India are Zerodha, Upstox, Groww or Angel One. They all offer fully paperless e-KYC/e-sign flows (Aadhaar OTP or video KYC), quick activation (usually 24–72 business hours) and mobile apps that walk you through the process. (support.zerodha.com)

A quick comparison (ease-of-opening view)

• Zerodha — fully online with Aadhaar e‑sign + video verification; account usually activated within ~24–48 working hours. Good if you want a simple, low-cost broker for active trading. (support.zerodha.com)
• Upstox — end‑to‑end online signup with Aadhaar e‑sign (OTP) and e‑signing; typical activation within a few business days. Simple, fast UI. (help.upstox.com)
• Groww — advertises 100% paperless Demat + trading onboarding via app/website; positioned for beginners (also free/opening offers at times). Activation typically in a few business days. (groww.in)
• Angel One — online account opening with e‑KYC; accounts generally activated within 48–72 hours. Good full‑service/discount hybrid. (angelone.in)

What “easiest” means in practice

• Paperless e‑KYC (Aadhaar OTP e‑sign or video KYC) so you don’t have to mail physical forms.
• Minimal documents: PAN, Aadhaar (linked to your mobile), a cancelled cheque or bank statement, photo/signature via phone camera.
• Few verification steps and fast activation (1–4 business days for most popular brokers). (support.zerodha.com)

Important requirements & gotchas

• Most online flows require your Aadhaar mobile number to be linked (OTP sent to Aadhaar‑linked number). If Aadhaar isn’t linked to your phone, you may need offline processing. (support.zerodha.com)
• These are for resident Indians. NRIs or people without Aadhaar/PAN must follow different (often partly offline) procedures. Check the broker’s NRI/alternate onboarding instructions. (support.zerodha.com)
• Compare AMC and brokerages after opening — “easy to open” ≠ cheapest to operate long term.

How to pick quickly

1. If you want the simplest, fastest signup: try Groww or Upstox (very guided app flows). (groww.in)
2. If you want a widely used low‑cost trader platform: Zerodha is a common choice (paperless e‑sign + video KYC). (support.zerodha.com)
3. If you want a broker that’s full‑service plus easy digital onboarding: Angel One. (angelone.in)

If you’d like, tell me whether you’re a resident Indian or an NRI and whether you already have PAN and Aadhaar linked to your mobile — I can then point you to the exact signup link and the shortest step‑by‑step for that broker.