Nobody's reviewed Gandalf yet? Is he just too obvious, or is he actually that unpopular??
Gandalf is one of the cornerstones of my favorite deck, which I call Three) Wise Men. Vilya is the other cornerstone, which in most decks is a hit-or-miss proposition. With Imladris Stargazer it becomes viable, but with Gandalf it is incomparably strong.
It's impossible to review Gandalf without a mention of Wizard Pipe, which can only attach to him or a small number of allies, half of whom leave the game at the end of the round. Once again, the Pipe is not a very good card in and of itself, but because of what you can do with it and Gandalf's ability. If you put a card on top of your deck that you can immediately play, you've effectively drawn the card you swapped it for.
It's also impossible to review this hero without mentioning Gandalf's Staff, which again is pretty much impossible use without him. I sometimes wonder why I'm paying 2 resources now for a card that will only get me on resource per turn, and it'll take two turns for me just to break even. Then I remind myself that I gladly pay 2 for Gléowine, in almost every deck I build, and the Staff will do exactly the same thing if I want it to. It also can discard a shadow card before it's revealed, which is handy in a pinch, and keeps it useful when all the cards have been drawn and played and there's no more need for more resources. It's also nice that Gandalf can pay his colorless resources for the Staff, which can then give resources to heroes of other colors, or even other players. This flexibility is a key difference from the normally stronger Steward of Gondor, and since Gandalf shows you what's coming, you can use that information to your advantage.
By now it should be obvious that we can't thoroughly evaluate this card on its own, but let's stop and take a look at the card itself. 14 threat is as high as it gets, and all I'll say to that is that you'd better make it worth it. You can see your next card on top of your deck, and up to 7 times a turn you can play it. Only once per turn can the card played be an ally or attachment, so this obviously tends toward an Event-heavy deck. He doesn't gain you any extra resources, so this leans towards events that are cheap, or preferably free. But, in a deck that's built for it, this is an incredibly powerful card draw engine. Every time you play off the top isn't just a card play, it's also a card you drew for free. For example, normally with Daeron's Runes you play one card, draw 2, and then discard one, so your hand size actually remained the same. Gandalf did all that without removing the card from your hand in the first place, so it was essentially a free draw of a card. Thus, Gandalf can draw up to seven cards per turn. Amazingly powerful! Almost completely unmatched! Peace, and Thought comes close, gaining 5 cards for a cost of 1 card, 1 resource, and 2 heroes exhausted just before the turn starts. That's not an insignificant cost.
Meanwhile, here we have the strongest hero statline in the game: 3/3/3/5. He's not the best quester in the game, but he comes close; not the best attacker, but though many can match him almost none will beat him; not the best defense, but he's still very close. He does whatever you need, making him an obvious target for things like Unexpected Courage. But let's take a look at another card that comes with him: Flame of Anor. This is an incredible chance to recover from certain disaster; in a tight spot, Gandalf readies himself and gains bonus attack for the phase. You can see the card you're about to lose and how much attack it will give you, or if desperately needed you can take a random discard by playing the Flame from your deck, again gaining a card draw. Many times it's actually irrelevant, if you just needed him to block something.
As my last word on the card itself, I invite you to read the text carefully. You may play the top card of your deck. When you do so, Gandalf is all colors. Nothing says that only Gandalf can pay for the card, so if you have for instance no resources on him and A Test of Will on top of your deck, you can still spend a resource from Glorfindel to play it. What could be better? The four colors give him even more flexibility for his effect, but I caution players against using too many cards that are off-color to their other heroes. Once you've drawn a card of the wrong color, it becomes a dead card unless you have some other way to play it, and the Wizard Pipe will only reactivate one such card per turn.
There are two more cards that I have to mention, and one of course is Shadowfax. Like the staff, this is really only viable on Gandalf or Gandalf, since Gandalf leaves the table at the end of the round. It's slightly more expensive than Unexpected Courage, but it makes him Ranged and Sentinel, and also can be paid for with any color. Assuming you're in a multiplayer game, and given Gandalf's amazing statline, the horse is usually well worth the extra resource.
And finally, more recently we have gained Narya for use with Círdan the Shipwright or Gandalf, the only Ring of Power with such a choice. It should be obvious that readying two allies for +1 and +1 is strong, and depending on the allies may well be stronger than Gandalf's own statline. This goes extremely well with the Vilya strategy I mentioned at the start, because expensive allies are strong allies, and thus Flame of Anor becomes even more powerful. Less obviously, the icon Gandalf gains from the ring makes him much less of a burden when trying to play cards already in your hand.
Obviously, there are costs to be weighed here. 14 threat is not cheap, and Gandalf remains one of the strongest allies we have, especially with Sneak Attack. When you decide to play Gandalf, your whole fellowship loses that opportunity, and I don't take that lightly. A lot of assumptions about deckbuilding have to be scrapped, if you're going to make full use of his ability. If you do, however, the results can be spectacular, and you will find that Gandalf could well be the strongest of all the Gandalves.