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.
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
The Black Knight
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!