I see these threads pop-up quite a bit, so I thought I'd write a quick-and-dirty guide to help newer players out on their crafting decisions.
As a F2P player who's saved up a lot of dust, I've thought about this list quite a bit, as any legendary card I craft is a great investment in time.
How to Acquire Dust
The best way to acquire dust is to play arena! Every once in a while, one of your reward boxes will contain a small amount (or a large amount if you're a good at arena) of dust that will help you get closer to your goal of crafting a legendary of choice.
I'd recommend players to only dust duplicate and golden cards, because you'll often find that dusting a rare or epic card for a class that you don't play will come back to bite you once you've built up more of a collection, and want to start branching out to playing other classes.
(This has happened to me a few times in the past. I'm looking at you Control Warrior!)
Please note that Neutral Legendaries can be used in any class, while Class Legendaries can only be used in a deck for a specific class.
I'd recommend that most people start with the Neutral Legendaries list, and then craft Class-Specific legendaries based on which class they like to play the most.
- Dr. Boom
- Fits into almost all but the fastest decks. Easiest way to populate a board that's empty. Certainly the best 7 mana minion in the game.
- Sylvanas Windrunner
- Another extremely flexible card that can be fit into almost any deck. Her Deathrattle is unique, and will often trade two or even three for one if you include the minion that she steals.
- Ragnaros the Firelord
- An 8 mana finisher. Best used when you've cleared the board to isolate a few large targets, or to end the game. This is a card that will fit into the late game (big dude) slot for most mid-range and control decks.
- A neutral legendary that is pretty essential in many control decks. Can go into Control Warrior, Freeze Mage, Handlock, and new Miracle Rogue among others.
Meta-Dependent Neutral Legendaries
The Meta-Dependent Neutral Legendary list is a nice to have, if the rest of your Neutral Legendaries collection is complete.
- The Black Knight
- Great in a taunt-heavy meta, the battlecry that allows you to destroy a taunt minion is unique in the game, and can give you a massive tempo-boost. Unfortunately, a bit weak when aggro decks are popular on the ladder, as when he doesn't hit the battlecry, he's a 6 mana for a 4/5.
- Primarily used in control decks to counter other control decks, as holding a situational card for a large part of the game in hand isn't as punishing.
- Harrison Jones
- Good in a weapons-heavy meta, and primarily used in control decks.
- A great card when the meta is super-slow and filled with other control decks, as she'll give you card advantage. However, in a ladder filled with aggro, she won't be effective, as you'll be dead before you're able to take advantage of her card generation.
Crucial Class Legendaries
These are pretty much must-haves for any deck that wants to play competitively for these classes.
- Grommash Hellscream (Warrior)
- Tirion Fordring (Paladin)
Nice to Have Class Legendaries
These aren't as crucial as the first list, but will improve your deck if you have the dust. Otherwise you can substitute them with a neutral legendary or rare/epic card at the same mana slot.
- Lord Jaraxxus (Warlock)
- Vol'jin (Priest)
- Neptulon (Shaman)
- Al'Akir the Windlord (Shaman)
- Cenarius (Druid)
- Archmage Antonidas (Mage)
Nice to Have Neutral Class* Legendaries
These are neutral legendaries that are used only in a few classes.
- Baron Geddon (Warrior)
- Bloodmage Thalnos (Mage, Rogue)
As of right now I only have Dr. Boom, Sylvanas, Ragnaros, Alexstrasza, Harrison Jones, and Grommash but I've been able to craft and play pretty much all the Control decks with a few substitutions.
For an idea of how this works, here's a list of the decks I'm currently running with only a few good neutral legendaries. I hope this helps, and happy dusting!