Not as complex as the newer versions, but by far my favorite in the series!
Harvest Moon 64 is my favorite in the series and I've played nearly all of them, barring any GBA or DS versions. The graphics are obviously a little old school, but not at all bad for a game of its generation. It's utter simplicity is what makes it so enjoyable. It's a lot simpler than the later versions, but still has quite a few challenges to overcome. The two biggest reasons I prefer this game to the others are: 1) the concept of time. Days aren't nearly as long, so you can play further in the game without taking your entire day to get there. Also, the fact that there's a two year deadline. You can certainly keep playing after that point, but you get a "score" by your frowning father after two years. 2) I really hate the stamina bar they've added in MM and ToT. While there is a concept of stamina in 64, it's not nearly as constricting as in later games, allowing more freedom. Some may say it's cheating, but given the shorter days, it all balances out in the end.
I also have a slight resentment towards ToT and AWL for taking out almost all of the characters. With many of the earlier games, the characters were often the same game-to-game. They may have had different names, but it was comforting to see a familiar face. 64 has many of the main, recognizable HM characters.
In short, Harvest Moon 64 is definitely my first choice when replaying any HM game. That's not to say I don't love the others, but this one certainly takes the cake. If you can get your hands on a copy, do so!