I'm playing FL and SoS, haven't tried RF4 yet, but I've heard and read a lot about it.
FL is a fun game with a simple happy story. There isn't much character development or personal struggle or anything like that, it's pretty much on the same tier as Kirby story telling (excluding the whole grimdark shit)
The mechanics are simple but fun, gathering materials, crafting shit and traveling around is pretty neat. It has it's own ideas and the whole schtick of the game is about being an errant [class]. By this I mean that you don't have a farm to tend or a kingdom you always go back to, you'll be traveling a lot all the time. Even as a cook, you're gonna head up a mountain to get some eggs from a legendary bird so you can cook an amazing omolette.
Get the expansion they added, it has a fuckload of extra content and you can give gifts to people for relationship points.
Story of Seasons is an odd one. Frankly, I'd recommend that you check out Trio of Towns instead since it's sorta the same but with more stuff.
It's harvest moon, a lot of planting stuff and selling, talking with people and doing festivals. However they went with some new ideas regarding renting fields to plant stuff and traveling shops instead of a shipping bin. It's different and that might not be good for some people until they adjust to it.
The waifus are top tier though.
Rune Factory 4 I haven't tried but here's what I can tell you. An anon in another thread said that he was about 600 hours in and there was still shit to do.
If longevity is what matters the most for you, go with it. There's farming, there's taming, there's combat, there's exploring, there's waifus and there's story.
All 3 games are great but RF4 is the most feature-complete out of them all and you can't go wrong with it.