I love Harvest Moon! It's always been one of my favorite game franchises. I've only played the original SNES version, Back to Nature, and Island of Happiness.
The original is probably my favorite. It's pretty simple compared to the others I've played but that's why I like it. Also, the graphical style was inspired by Legend of Zelda. It was also the first HM game I ever played. I used to rent it constantly as a kid. The rest of my family told me the game looked boring and they couldn't understand why I liked it so much.
BtN is great too, though there's so much to do in that version that I feel overwhelmed sometimes. Last time I played it, I actually had to write out a daily schedule for the game! I couldn't really get into IoH. I like the whole shipwrecked story, but I found it way too hard to able to enjoy it. You can hardly do anything without your character passing from hunger/exhaustion.