One of my favourite heroes. I might say actually a bit under-rated as well, though some might dispute that - it seems to me though that some people find it too difficult to look past that 14 threat cost and blocking of Core Gandalf and see the huge variety of options hero Gandalf can offer. If you put the effort into making him work though, I would say Gandalf is absolutely the most versatile hero in the game - there's nothing he doesn't work well with, except maybe Secrecy. Even then, shenanigans can be pulled.
So our first port of call is his stats, the best of any hero. Gandalf is an excellent choice for any of the three primary roles of quester/attacker/defender with 3 in each stat and 5 hit points. A lot of decks will want to maximise the use of those stats using readying effects such as Unexpected Courage and of course Shadowfax and obviously it's worth doing. Further to this another one of Gandalf's own particular cards comes into play for this in Flame of Anor, which can vary from just readying him to use his already formidable base stats to potentially readying him with a whopping +6 if you discard Beorn or Brok Ironfist from the top of your deck (probably the best use you'll ever find for Brok Ironfist). That's a pretty big deal for handling combat, but of all the cards specifically aimed at Gandalf I find it the least useful by a significant margin, which I think says something about the others. Likewise for all that his stats are amazing, they're not the true draw of the hero to me, just an incidental benefit. Yes, 14 threat worth of stats are an incidental benefit to me.
The real appeal of hero Gandalf is that ability. At first glance it might not seem so amazing - essentially your hand is just always one card bigger, right? But the thing is, any time you play that card from the top of your deck it is of course immediately replaced by another, new, top card of your deck. Thus effectively drawing you a card every time. If you have a stack of affordable events (perhaps set up by an Imladris Stargazer) you can do this multiple times per round, but even confined to the Planning phase it's pretty amazing - effectively drawing an extra card potentially every single round, not that easy an ability to come by usually. And as if it weren't enough that Gandalf's ability functionally is card draw, it also helps with more normal card draw in the form of Expert Treasure-hunter since you always know what to guess. And that can further be combined with Hidden Cache to generate extra resources as well. This is amazing, and there's still more. While playing a card this way, Gandalf is considered to have all four regular printed resource icons. The fact they're considered printed has minor benefit with some of the Against the Shadow mono-sphere cards; the fact he has the icons lets you play some attachments on him you otherwise couldn't, like Armored Destrier, A Burning Brand, Silver Lamp, or Support of the Eagles. The best aspect of it in this case though is the obvious - that he can play any card of any sphere, not only making his otherwise Neutral resources more useful, but also giving scope for decks using Gandalf to splash in any card they like that really fits the deck but doesn't match spheres with the rest. The applications of this are much the same as the comparable applications of A Good Harvest. Even with the amount of sphere bleed which has occurred over the years it can still be very nice to just slot in one or two cards you otherwise couldn't, depending on Gandalf to play them. Note also that since Gandalf is considered to have the icons he can play 0-cost cards like this, unlike Elrond's ability to play out-of-sphere allies.
If that were it, Gandalf would be interesting but very tricky, with his effectiveness rather depending on the shuffle of your deck as to whether a useful card was on top at the right time. But then of course they also gave us the ultimate enabler for him in the Wizard Pipe. There's little more to say about the Pipe other than the fact it can be conveniently fetched by Bilbo Baggins - it does nothing of itself, but it makes Gandalf's ability massively more reliable, and that ability is amazing as I've just been saying. Without the Pipe there'd be inevitable fear of drawing your out-of-sphere cards, but with the Pipe it's no problem as you can just stick them back on top of your deck again from your hand. The one additional point I can bring up is that it makes a ridiculously strong combo with Elrond and Vilya.
Now an important thing to note is that despite his big stats, Gandalf's ability - sphere-smoothing and effective card draw - is a support ability rather than one about his personal power. This is where I think a number of people fail to see the potential in hero Gandalf - they look at the statline and reason the only way to make use of him is to load up on readying and focus everything on turning Gandalf himself into a powerhouse. Now this is a valid approach to take, but it's only one of so many possibilities, and I feel much more interest can be derived from the other possibilities, where Gandalf's personal power is of less significance and he's more just a support hero, using that ability to enable something else. Admittedly he's a 14 threat support hero which can present problems, but he makes everything work so much better I'd say it's worth it. His position as a 14 threat support hero is solidified by a couple more of his toys - Narya obviously is great for any setup which has some powerful allies who will benefit from extra actions, while Gandalf's Staff is just good in any context, adding to the smoothing and draw Gandalf already provides by himself with the addition of flexible resource generation and card draw (two absolutely central mechanics to deck power); and just as a bonus you also have the option to take some of the risk out of your game by discarding shadow cards. I fully intend to review both of these cards properly so I won't go into too much detail, but the basics are enough. Despite his high threat cost and stats, Gandalf's biggest impact on a game can be in a supportive role. One further point I will note about all of Gandalf's toys is that they all have the advantage of being Neutral like him, making them easier to play than they otherwise would be.
The one non-Neutral Gandalf card is one which preceded his release - Word of Command. I've already reviewed that card, obviously it's very powerful, though exhausting a hero as powerful as Gandalf certainly can be a wrench. It's not an option that I necessarily use too often, but it can make Gandalf an excellent choice for a deck which depends on some particular combo to work properly.
The final point I want to bring up is this: I've mentioned a lot of cards which work very well with Gandalf, many of which were specifically designed to work with him. However I shoul stress that just because these cards work well with Gandalf in no way obligates you to use them with him. I've seen people complain about the number of 'required cards' for a Gandalf deck being too high, but personally, the only ones I never omit are the Pipe and the Staff - because the Pipe makes Gandalf's ability work so much more reliably, and the Staff just makes a deck work so much better in my experience. Everything else is optional. Sure, if your focus is to get as much out of Gandalf as possible then you'll probably want a lot of them, but if instead your goal is to just have Gandalf enable some other idea then you take those basics and any others which fit with that idea, which might be none of them.
Decks I've used hero Gandalf in vary between Vilya, Secrecy shenanigans, Pipes, Silvans, Glóin, and Dúnhere; and I know I've barely scratched the surface of what he can do. There are just so many possibilities, so many opportunities for creative deckbuilding from a hero who can do pretty much literally anything that it's possible to do in this game. Gandalf is every bit as interesting as he should be by rights. He is a wonder.