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.

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.

The Meta-Depentent Legendary list is a nice to have, if the rest of your Neutral Legendaries collection is complete.

Neutral Legendaries

1. 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.
2. 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.
3. 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.
4. Alexstrasza
   • 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.

Crucial Class Legendaries

• Note: These are pretty much must-haves for any deck that wants to play competitively for these classes.

• Grommash Hellscream / Tirion Fordring / Al'Akir the Windlord

Nice to Have Class Legendaries

• Note: 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 Jaraxxas / Vol'jin / Cenarius / Baron Geddon

Meta-Dependent Legendaries

1. 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.

2. Harrison Jones

   • Good in a weapons-heavy meta, and primarily used in control decks.

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.

I hope this helps, and happy dusting!