This is probably the question I get asked the most. The answer is quite simple - any time!
Weather - the best weather Iceland has to offer, "warmer" temperatures, less precipitation
Activities - puffins, hiking, whale watching, glacier hiking, paragliding, lupines
The seas around Húsavík, in North Iceland, provide the best whale watching opportunities in the country. North Sailing is an incredible company that offers unique experiences like sailing on an old wooden ship or gliding silently across the water on their electric, carbon neutral boats.
Crowds - the most crowds, you're unlikely to be alone
Daylight - never-ending!
Accessible Regions - everywhere!
Although Highland roads (F-Roads) are open, you MUST still have a 4x4 vehicle to drive on them. Since not all F-Roads are the same and not all 4x4s have the same capability for navigating them, your best option is to rent a Super Jeep, a modified 4x4 with a snorkel from ISAK. This vehicle will get you across nearly every river crossing without drowning the engine and will be a much smoother ride over large stones and potholes.
Best types of trips (in general) - trips of any length, camping or staying in hotels, 10 or more days driving the Ring Road, add at least another week to explore the Highlands and Westfjords
Weather - decent weather & fall colors to the beginning of winter weather, maybe some snow
Activities - puffins are gone but Northern Lights return, some ice caves are closed, hiking is good, glacier hiking, whale watching, blue ice caves begin to open in October (this depends on glacial melt and they may be open in the summer too)
Touring an ice cave is a must-do activity while visiting Iceland but most natural ice caves are unsafe and inaccessible in the warmer months. However, Katla ice cave, near Vík, in South Iceland is open year-round! Because the ice is layered between volcanic ash, it doesn't melt as quickly and the surrounding landscapes are beautifully dramatic.
Crowds - shoulder season, crowds are thinning out
Daylight - enough daylight to explore
Accessible Regions - most everywhere but Highland roads begin to close in mid-September and usually all F-roads are closed by mid-October.
Best types of trips (in general) - 10 or more days driving the Ring Road, camping or staying in hotels, add at least another week in early September to explore the Highlands and Westfjords
Weather - lots of white snow, below freezing temperatures, challenging storms which may make it unsafe to drive & explore
Activities - decent time to see Northern Lights and do winter activities (dog sledding, snowmobiling, skiing), hiking is not always realistic as trails are covered in snow and ice
Is seeing the Northern Lights at the top of your Iceland bucket list? Icelandic Roamers provides small-group and private Northern Lights tours with professional photos throughout the Aurora season. Let them do the driving for a better chance at seeing the lights safely and they'll capture amazing images for you to remember.
Crowds - the least amount of crowds (except around the holidays)
Daylight - 8 hours at the beginning of November to just 4 hours near Christmas (winter solstice), then we start gaining daylight and are up to 13.5 hours by the end of March
Accessible Regions - Reykjavík, Golden Circle, Snæfellsnes and Reykjanes Peninsulas, and the Ring Road. Even though all of these areas are open for travel, beware of challenging road conditions throughout the country in the winter.
Best types of trips (in general) - 5 or more days, based out of Reykjavik or spending multiple nights in the same hotel in different regions, taking day trips to explore, sticking to the South Coast and western regions (not the Westfjords)
Weather - cold weather but also decent, snow is still around in April but only for a few more weeks, transition to Spring
Activities - the last few days and weeks of Northern Lights, puffins arrive, some ice caves begin to close, hiking trails become safe again but can be muddy, whale watching is good, and boat trips on the sea and glacier lagoons open in May
Even though Highland roads are closed in the spring you can still get a taste for the gorgeous scenery on a Super Jeep tour with Midgard Adventure! Throughout the year Midgard can take you across rivers, into canyons and through the hiking paradise of Þórsmörk in their picture-worthy, massive and comfortable Super Jeeps.
Crowds - fewer crowds than summer but starting to pick-up
Daylight - plenty of daylight for exploring
Accessible Regions - most everywhere except the Highlands. The Westfjords are also accessible in late Spring but many services (restaurants, tours, museums) are still closed.
Best types of trips (in general) - 7 or more days, spending multiple nights in the same hotel in different regions, taking day trips to explore, sticking to the South Coast and western regions, Ring Road is also an option with 8 or more days, camping in May
Go check out my guide for the best free things to do as well as itineraries and travel tips to make your trip unforgettable.