Yeah I know the thread title sounds sad, but whatever. I'm not interested in playing this games with friends yet. Just wanted to know if any of these games are worth trying out alone, having a good single player experience and such.
Why not also make up some rules along the way? I know my solo experience in Mario Party 6 was way more fun than most people's solo experience in Party Mode because I was using another controller to try to make other CPUs duel Wario for coins (since he usually doesn't have stars), force Wario to refuse stars, and lose on purpose with 2 v 2 minigames where Wario is your partner. It also helps to have a friend watch and react to stuff.
Mario Party Advance has a pretty decent single-player campaign, as it's dedicated with a single player in mind and I think it has lots of fun little quips and interactions with a wide variety of Mario characters. It doesn't really play like a traditional Mario Party game as well, being more focused on completing those tasks than playing minigames or buying stars or competing with others.
Mario Party DS has a decent amount of collectibles to focus on collecting, so I enjoy that as well, even if completing the Story Mode with all six characters can be tedious if you want to unlock everything.
If Mario Party: Island Tour had just Bowser Tower and scrapped its bad boards to focus just on that, I may have sorta recommended it as well, but as it is, I think it's too bare of a game to recommend to anyone (it doesn't have a bunch of collectibles worth collecting like DS does) and it contains awful board mechanics regardless that boil down to luck.
Most Mario Party single player modes are just, beat the CPU in a bunch of boards with traditional Mario Party rules, and some of them such as Mario Party 4 require you to do through with each character if you want to unlock all collectibles. It's pretty difficult recommending one with a single player in mind since that's not the series' focus.
Go for Island Tour, it's my favorite Mario Party of the ones I've played and is great for solo play. Then again, its best board (an awesome board game variation of Old Maid) is locked to multiplayer, so maybe not. At least Bowser's Tower exists.
I haven't played DS but I've heard good things about its story mode. Top 100 also has a good one, but considering that that's just a mediocre game all around, I wouldn't recommend it.
I know it's been mentioned a bunch in this thread, but Mario Party DS is great. Being a DS game, it was designed with single player in mind. It's got a great story mode, a bunch of fun side modes, a ton of collectibles, and lots of fun minigames. My favorite thing about the game, though, is the theme. I love how all the boards and minigames are designed around being small. It really ties the game together and sets it apart from other games in the series. I'd have to say it's one of my favorites.