I know this has probably been asked 1,000 times, but...

[tha bugman]

Primos True Double

[TnTurk]

Easy, long box - sound like half dozen or more birds out of one call, and will do anything from the kee-kee to a gobble.

100% agree. A "good" longbox can make any sound that a fella would need to call in a big gobble.

[chatterbox]

Easy.

Andy Kaiser double sided glass/ slate with waterproof purpleheart striker.
Does everything from soft purrs and clucks, to mating yelps, and cuts.

[GobbleNut]

If I had to choose a single call for the rest of my days, it would be a pot with a slate surface and a carbon striker.  If I had to choose a single type of call, it would be a mouth call,...under the condition that I could use a new one of the same type when it wore out.

[LaLongbeard]

I've killed with about every type made but if I had to use just one it be an old walnut scratch box call I've had since the beginning. Its not fancy and not loud but it is all hen.

[USMC0331]

Trumpet.  Since learning how to run one a few years ago it is "The 1 Call" .