Great question — “ultimate” beach depends on what you want (surfing, whale‑watching, snorkeling, luxury seclusion or family‑friendly sand). Below is a short, practical guide to the best places to stay in Sri Lanka, when to go, what to expect at each spot, and quick planning tips.

Quick season summary (why it matters)

• Sri Lanka has two monsoon patterns: the southwest coast (south & west beaches) is driest and best from about November–April; the east coast is best from about May–September. Plan your beach region around these windows for calm seas and sun. (thetimes.co.uk)

Top beach areas and why to pick them

• South & Southwest (Mirissa, Unawatuna, Hikkaduwa, Galle, Tangalle)
  • Best for: whale watching (Mirissa in season), sunsets, mix of lively beach towns and quieter coves, luxury resorts and boutique villas. Ideal November–April. (mirissawhalewatchingsrilanka.com)
• West coast / family resorts (Bentota, Kalutara, Negombo)
  • Best for: family‑friendly beaches, water sports, easy access from Colombo (good for short stays or start/end of trip). (thetimes.co.uk)
• East coast (Arugam Bay, Pasikudah/Kalkudah, Nilaveli/Trincomalee)
  • Best for: world‑class surfing (Arugam Bay — peak May–September), long calm bays and snorkeling (Pasikudah, Nilaveli/Pigeon Island). Visit May–September for best conditions. (lonelyplanet.com)
• Lesser‑crowded/quiet options (Tangalle, Talpe, Goyambokka)
  • Best for: privacy, photo‑perfect secluded bays and boutique luxury, great for honeymooners and quiet retreats. (planetware.com)

Must‑know activities tied to locations

• Whale watching: Mirissa (south) — peak season November–April (best months Dec–Mar). Many operators run dawn trips when seas are calm. If you travel outside that window, Trincomalee and parts of the east can be good when whales migrate east. (mirissawhalewatchingsrilanka.com)
• Surfing: Arugam Bay (east) — best May–September; Hiriketiya, Weligama and Midigama (south) have surfable breaks in the opposite season. (lonelyplanet.com)
• Snorkel/diving: Nilaveli / Pigeon Island (Trincomalee), Hikkaduwa reef, Passekudah/Kalkudah. (lonelyplanet.com)

How to pick where to stay (short checklist)

• If you want whale watching and nightlife + good restaurants → stay Mirissa or Unawatuna (south). (mirissawhalewatchingsrilanka.com)
• If you want surf culture and laid‑back guesthouses → Arugam Bay (east) or Hiriketiya (south). (lonelyplanet.com)
• If you want calm, shallow swimming and resort comfort → Pasikudah / Kalkudah (east) or Bentota (west). (thetimes.co.uk)
• If you want luxury seclusion (villas/resorts) → Tangalle / Goyambokka / boutique properties along the south coast. (planetware.com)

Two sample “ultimate beach” approaches

• South Coast luxury + activities (best Nov–Apr)
  • Base in Mirissa or Tangalle. Do whale watching (Mirissa), a day trip to Galle Fort, surf lessons or snorkeling, then retreat to a private villa or 5‑star resort for spa/dining. (mirissawhalewatchingsrilanka.com)
• East Coast surf + long beaches (best May–Sep)
  • Base in Arugam Bay for surf breaks, then hop north to Pasikudah/Kalkudah or Nilaveli for calm bays and snorkeling. Good if your priority is surf and quiet long stretches of sand. (lonelyplanet.com)

Logistics & practical tips

• Getting there: Most international flights arrive at Bandaranaike/Colombo (CMB). South coast (Mirissa/Bentota/Galle) is 2–4 hours by car (Southern Expressway speeds up travel); east coast (Trincomalee/Arugam Bay) is longer by road or you can use internal flights on some routes. Expect transfers to take longer during peak season or holidays. (destinationsrilanka.travel)
• Book activities seasonally: whale and boat operators run mainly in calm months; surf seasons are regional — check operator schedules and book in advance in high season. (mirissawhalewatchingsrilanka.com)
• Safety & advisories: check your government travel advisory immediately before booking/going (situations can change regionally). There have been occasional localized security notices in the past (for example some advisories affecting visitation to parts of the east/coast at times), so confirm current guidance. (apnews.com)
• Accommodation style: Sri Lanka has everything from budget guesthouses and surf camps to boutique villas and multi‑star resorts. If you want “ultimate” comfort, look for beachfront villas or 5‑star resorts on Tangalle, Galle/Mirissa or upscale properties in Passikudah/Pasikuda. (planetware.com)

Quick recommendations (one‑line)

• Best for whale watching + lively beach scene → Mirissa (Nov–Apr). (mirissawhalewatchingsrilanka.com)
• Best for surfing culture → Arugam Bay (May–Sep). (lonelyplanet.com)
• Best family / easy arrival starter beach → Bentota or Negombo (close to Colombo). (thetimes.co.uk)
• Best secluded luxury beaches → Tangalle / Goyambokka (south). (planetware.com)
• Best snorkeling / calm bays → Nilaveli / Pasikudah (east). (lonelyplanet.com)

If you want, I can:

• Suggest 3–5 specific hotels (luxury, midrange, budget) for the exact dates you’re considering (I’ll check availability and current prices), or
• Build a 7‑day or 10‑day beach itinerary based on your travel dates and interests (surfing vs. relaxation vs. wildlife).

Which would you prefer?