A MacGuffin was originally a film concept, defined as "an element that drives the plot of a work of fiction." Basically, they are cheap ploys that make a hero want to do something, and they've never enjoyed more prevalence than in the video game industry. Why do you want that princess, star, or weapon you can't even use? Because you do, you benighted controller monkey! Here's our tribute to the MacGuffins that deserve recognition for being awesome, terrible, ironic, or just confusing.
The Triforce was left behind by three goddesses (supposedly) and has the power to grant wishes, but only to a person with equal measures of courage, power, and wisdom. This seems to be one of those built-in after school special messages, as someone who already has all three qualities wouldn't have any trouble getting anything they wanted. Or even if you just had power, that's probably enough to put you on the road to success sans a set of wish-granting golden triangles. Of course, if you have no wisdom, power, or courage, presumably that's still equal measures, so we can only guess that Link is forever on a quest to prevent stupid, cowardly, unimportant people from touching the Triforce.
What would have happened without it: Ganon would have been forced to ruin lives by some other, less elegant method, like forming a standing army of desert people who are not so afraid of Master Swords, pretty ponies, and wind instruments. This situation would be moderately less difficult to navigate than the Water Temple.
Imagine you are designing a game about one woodland creature saving other woodland creatures. That motivation, on its own, isn't what we would call "compelling." Especially when you're never further than a cartridge change from some really brutal, violent, gritty game. Like FIFA '95. So to motivate the story, you introduce a set of collectible objects with "mystical powers" that are the source of anything good thing that can happen to you. Make your hero collect some non-obvious number of them, like seven. Leave the subtlest of hints that these objects powering the game are radioactive by making the world have an infinite supply of two-tailed foxes.
What would have happened without them: Robotnik would have kept possession of all the woodland animals from all the Sonic levels, so the game developers wouldn't have had to animate them bouncing out of their cells throughout the game. Way to make more work for your employees, Sega.
In the giant, intricate, confusing universe that is Mass Effect, everyone wants one simple thing: the Conduit. Trouble is, no one can agree on what it is. Some say a weapon, some say a big humming art sculpture. Your team of space commandos spends most of the game wondering about it until the bad guy, Saren, uses it to relay himself into the Citadel to mount a surprise attack. Oh. I guess that is what a "conduit" would do. Note to Bioware: it's not that surprising when the secret purpose of your MacGuffin is its own dictionary definition.
What would have happened without it: The Protheans originally needed the Conduit to sneak onto the Citadel so they could shut down the mass relays the Reapers used to murder every sentient race in the galaxy. Without it, well
no one was expecting the willowy, sophisticated race to win out over giant tentacle monsters anyway. If I've learned anything from sci-fi games, H.P. Lovecraft, and insane Japanese porn, it's that tentacles conquer all.