Definitely the one that interests/attract you & your ear.

In terms of my exposure ...I chose bass because a high school band needed a bass player...I had to buy it on "lay-by" across three months to cover the full $75 in 1974. It kept me very happy until I'd played in a couple of live groups and the need for a full scale bass became apparent.

Bass is easy to start on, relatively easy to get okay at but hard to master. I have five and none cost more than AUS $400 but the 2nd I bought, $112 dollar newie in 1980, is def. worth more.
From there I added:
Guitar is easy to start on, fairly easy to get so so on, (I survived writing with just a dozen chords for twenty or so years), and hard to master.I've not spent more than $375 on one but I do have a more expensive one, (an insurance replacement for an old thing that couldn't be directly replaced), that sits in its case.

Cello is easy to start on WITH a tutor and impossible to master. They can be bought fairly cheaply though getting the bridge set up, fine tuners and such will add a bit to that.

Piano/generic keyboard is easy to start on and really hard to master. The world is filled with cheap and cheap sounding keyboards to learn on - my wife had a 110 year old parlour grand that was lovely but preferred her cheap knocked about upright.

These days a stringed instrument and a tuner will get almost anyone started...a tutor makes an huge difference but the RIGHT tutor is the issue.
The downside of a uke or banjo is the effort required to transfer chord shapes to guitar/mandolin etc.


BUT if you do want to write songs something you can play chords with will help as you'll have "instant harmony".
I'm a poor player on bass, guitar, cello etc. BUT I enjoy them all and they all help me write & record songs.

Cheers
rayc
"What's so funny about peace, love & understanding?" - N.Lowe