Capsize: Buyback is a pretty polarizing mechanic. Cards with buyback either suck because they are over costed, or are super powerful. Capsize falls in the latter category. I have fond memories of getting infinite mana and bouncing back everyone else's permanents to their hands (although I'm sure they don't quite view the situation quite as fondly). Even if you aren't doing anything sick like that, its still a top notch blue defensive spell. If you're on the receiving end of a capsize, trying to play around is almost impossible, and you often are resorted to killing your own creature in response just so that the capsize gets countered and they can't buy it back. By the way, if you don't have capsize in your blue EDH decks you're doing it wrong.
Constant Mists: Continuing our theme of annoying buyback cards, we come to Constant Mists. You would think the buyback on this card would make it unplayable, but you really want this card in the late game, when you have a bunch of excess lands you aren't doing anything with anyways. Its so hard for an aggro deck to try to win through a constant mists once you get it going, and you'll usually be able to kill them before you run out of lands to pay for the buyback. Unlike capsize, there is nothing your opponent can do to play around it short of countering it. As with capsize, this card should be in your EDH decks if it can.
Fire/Ice: Once upon a time, Alex Gord had me redo a blue/red deck of his, and I changed it to this weird Counterburn deck that had 4 Fire/Ice and 4 Prophetic Bolt. The deck was a blast to play, but to my surprise at the time, Fire/Ice was the best card in the deck. It may not look like much, but theres never a situation where it isn't useful and I was almost always glad to draw it. Its been in my U/R decks ever since. Also, its a fun card to imprint onto Isochron Scepter.
Firestorm: This card is more of a shout out to Josh than anything else, although it is one of the best red sweepers ever printed. It also only costs one, so its pretty high in the surprise factor. What you trying to kill me?! Fine. I firestorm, discard my hand, and deal 7 to each of you and kill all of your creatures. Self-destructing is always a hilarious way to end the game. On a more serious note, combo this with Necropotence.
Gift's Ungiven: This is one of those fun cards that gives you opponent the illusion of being able to hinder your plans, when in reality they are fucked no matter which two cards they decide to give you. My favorite combination is to grab: Mindslaver, Academy Ruins, Life From the Loam, and Moment's Peace. Doesn't matter what your opponent picks with that combo, since they're going to get Mindslaver locked within the next few turns anyways.
Hatred: One of my favorite 'opps I win' cards. Seems really really good in EDH since you can one hit kill with a general or with an infect guy pretty easily. Richard used to have a deck that was designed around trying to kill his opponent on the first turn by casting Hatred (using dark rituals) on a Raging Goblin. Fun times.
High Tide: This card seems pretty innocuous at first glance, but blue really shouldn't be able to have mana acceleration this powerful. The card itself, wasn't very good until Urza's Saga, when the designers decided to give blue an untap lands mechanic. One of my favorite infinite mana combos of all time, is using High Tide alongside Palinchron and six islands. Even casting something like Time Spiral or Turnabout after a High Tide is usually good game.
Meditate: This card taught me a very valuable lesson about Magic growing up: It doesn't matter what the drawback of a card is if you can win the game after you cast it. This card is usually found alongside High Tide when trying to do a storm combo. Storm combos are when you cast spells over and over and over again in order to kill someone with a storm card like Brain Freeze, which gets copied for each other spell played that turn. Usually takes the form of the following: I cast High Tide and tap my remaining 5 islands for 10 mana. I use 4 of that mana to Turnabout my lands and tap them again, leaving me with 18 mana. I Meditate to draw four cards, cast a couple Impulses, then a Time Spiral, which untaps all of my lands and gives me a new hand. 20 minutes later (still in the same turn), the game is over with a giant stormed Brain Freeze. Fun for me, not so much for anyone else playing.
Momentary Blink: 'Blinking' is one of my favorite mechanics in the game of Magic. While it looks useless at first glance (I'm sensing a theme here), it does a couple of important things: It can save a creature from removal and it can re-trigger 'enters the battlefield' abilities. That last part is particularly important, since creatures with 'enter the battlefield' abilities tend to be quite good and blinking is the easiest way to reuse them multiple times. Favorite blink targets: Eternal Witness, Venser, and Avalanche Riders.
Remand: Best Counterspell Ever. Well not really, that title goes to Force of Will, but Remand is my favorite counterspell to be sure. Why is this spell so good? Think about what you do during your average turn. You untap your lands, draw a card, and maybe cast or attack someone. When you Remand someone's early play, what did you just do? You drew a card and stopped them from casting something. Its effect is very similar to a Time Walk. Granting you didn't get to untap your lands, but you did basically just take an extra turn. Easily one of the best 'tempo' cards ever printed. Too bad it sucks in EDH.
And there you have it, my favorite Instants of all time. I would pick High Tide as the winner for me in this category. Next up: Creatures