IF I HAD JUST ONE BIKE, it would have to be a hardtail MTB. Because in the words of Greg Focker, you can ride anything with wheels... but, it's where you can ride them and how fast they go that matters.
On a hardtail...
you can pump up the tires to rock hard and keep up with most road bikes,
you can run low pressure in your tires and climb up nearly any trail
or bomb any descent kinda fast,
you can have the air just right and go from sand to dirt to road and back,
you can be pretty good at almost anything.
Which is why I have a hardtail and a road bike;
and have my eyes on
a trail bike
a gravel bike
a tandem road
a tandem MTB
an old Schwinn Apple Krate
and an electric beach cruiser.
What are your eyes on?
Is your FUNKR team ready?