Easiest way to play podcasts?

Spotify, but the syncing is awful and manual

Definitely Spotify. Doing manual uploads using Garmin Connect is a PITA, even moreso than Spotify.

Do I need to have a paid Spotify subscription or can I do with a free account?

What about iHeart (Im in Canada and havnt been able to get the 6 yet so cant comment)? 

This was discussed in one of the recent DCRainmaker podacsts and Spotfy was the easiest method, though I think you do need to have a paid Spotify subscription to make it work. You can also manually copy files over. That would be the lowest cost solution but would be a pain.

Yes, you need a paid subscription.

I don't know if it is the easiest, but I have just resigned myself to manually syncing them using Desktop version of Garmin Express and drag-n-drop the MP3 files.  Runcasts was very flaky on my 5+ so I didn't bother installing it on my 6X.

There is no, easy, way, and if there is, it’s 100% not Spotify.  It can take 10 minutes to get a single podcast over to the watch with all the hoops you have to jump.  And you have to have a paid subscription.  One more thing, don’t listen to some history kind of broadcast where you have to start at the beginning or middle of 300 plus podcasts.  You have to go through 5 episodes at a time to find it.  My F5+ would crash around 100, haven’t tried it on the 6x.  I uninstalled Spotify, in fact.

I use Apple podcasts, and the Garmen Express app to transfer them via the power cable.  It’s not too bad but trying to get Apple podcasts synced to what I’ve listened to is painful.  It’s just not as painful as Spotify’s mess.

We really need Overcast or something like it.  Overcast synced across my apple devices, reliably updated overnight, particularly the watch, and it was free.  It was good enough that I wouldn’t have minded paying for it.

Perhaps Amazon music? Haven't tried yet but if you have Amazon prime, are there any podcasts there?

I used Garmin Express and I found it to be quick and painless.  Here's my workflow:

Mac with iTunes as my podcast manager.  I fire it up and let it download the latest for about 20 min or so.  I usually subscribe to mix show podcasts (Steve Aoki, Morgan Page, etc) so that I have a solid hour of music to run or workout to with each podcast.

I charge my Fenix6 once a week (usually monday morning at work while i'm sitting anyway).  Plug the Fenix6 into the laptop to charge, fire up Garmin Express, choose a handful of mix shows to sync and away it goes.  The watch is typically fully charged before lunch and the podcast sync only takes a couple of minutes.

Done and done.