Druids are one of the strongest classes in the game just thanks to their spell list alone. This gives you a lot of freedom with your subclass selection, which is fantastic as there are plenty of great options here. Unlike most classes, I actually had trouble determining a clear front runner, so don’t take my word as gospel here.

Druids are D&D hippies with a legacy of weird restrictions, broken summoning magic, and the power of shape changing in the form of Wild Shape. 5th Edition druids have some problems, but especially with some of their newer subclasses can be a wild and wonderful experience at most tables.

The fantasy of a druid in tune with the natural burning and regrowth cycles of life appeals to me, and mixing that with the Wildfire Spirit and its Fiery Teleportation and expanded spell list leads to a slippery character running next to their raging spirit.

Circle of the Shepherd is the dedicated summoning support option available to druids; because summoning is one of Druid’s strengths anyway, Shepherd by default will be doing powerful things. Its low-tier features are excellent, and while Mighty Summoner has some issues, the remaining features make up for the middling middle levels. 

Circle of the Moon is complicated, messy, and controversial, but can be a blast to play with specific expectations met by all parties involved.

The bread and butter spell-casting focused druid printed in the PHB is Circle of the Land. It predominately is a list of “domain” spells, a mirror feature to Wizard's Arcane Recovery, and some largely flavorful features with little to no impact on the game beyond that. 

Circle of Stars has never connected to me as particularly druidic, but it does an excellent job selling the constellation-based magic we see in witchcraft and ancient stories. It takes the majestic sky above and presents the user as a vessel for its majesty.

Circle of Spores gives druids something new to do, in a way that still feels druidic. It takes undead and meshes them into the natural feeling druids evoke, with the flavor being corpses animated with fungal life instead of evil undeath.

Druids, the most tied to nature and fey magic, come with Circle of Dreams as their fairy subclass. Their 2nd level feature is fine, but with them entirely lacking in a reasonable 6th level option you’ll feel pretty terrible up until you get Hidden Path to at least feel like a fey spellcaster