Story of Seasons occupies an interesting position in the farm-and-life-sim genre. It is different from Harvest Moon, but not all that different. That's because it actually is Harvest Moon, thanks to the confusing name change and company swap mentioned earlier.
Either way, what it primarily does is expand on everything old HM (when it was HM still) already established. For example, in the original Story of Seasons, you get more characters, more personality, more crops, more stuff to do, and more animals.
It also tries to hearken back to the olden days of brutal farming with a sharp difficulty curve that makes money-earning very, very challenging indeed.
Trio of Towns takes it further, trying to improve on the Tale of Two Towns concept by, again, adding yet more. The towns are bigger, the themes in each are more pronounced, there are even more animals, people, and crops, plus you have a ton of activities to do outside of farming.
Unfortunately, it doesn't really establish an identity for itself other than the more colorful, playful Harvest Moon. That doesn't mean the games aren't fun, because they are, but it takes more than loading on content to create a champion game.
Take the characters, for example. Trio of Towns falls into the same trap Tale of Two Towns did. There are more places to go and people to see, but each is fairly limited because of that.
Stardew Valley's characters might not be the most detailed and complex ever. However, they're interesting all the same, since they each represent a specific personality trait or life problem that resonates with people — even Shane the chicken man. Sure, they'll say the same line for an entire season, but you want to learn more about them.
Stardew's other mechanics are fairly basic when you think about it. However, they work well together. Mining is simple, but serves an important purpose both in expanding your farm and getting special gifts; there aren't 50 million crop varieties, but you can do a lot with a few things and shape your farm — and life — around just a handful of crops.
In some cases, simple is just better.
Comparing these games and series isn't an easy task, since they're all top rank in what they try to do. With everything Stardew Valley has to offer, though, with the promise of even more single and multiplayer content to come, that's the one you'll find me going back to every time for a bit of quality farm fun.