Hearthpwn

Aside from the legendaries in the Adventures, the only major legendary I have worth mentioning is Sylvanas and I'll admit I don't even run her in most of my decks.

I don't really play control decks (mainly because most of them are loaded with legendaries I don't own). I usually end up playing aggro or tempo.

The classes I play, in order of most played:

1. Warlock, Hunter, Shaman
2. Druid, Paladin, Mage
3. Warrior, Priest, Rogue

Are there any recommendations as to what my second major legendary will be? Some cards I've been looking at

• Ragnaros, Firelord (for general purpose?)
• Leeroy Jenkins (for my aggro decks)

Dragons (For a finisher card in Paladin / Priest / Warrior dragon decks)

• Alexstrasza
• Ysera
• Onyxia
• Deathwing / Deathwing, Dragonlord

Old Gods

• Twin Emperor Vek'lor (for C'thun Druid)
• Yogg (for potential Arcane Giant spell decks)
• Y'saarj
• N'zoth

Class Legendaries

• Jaraxxus (I play Warlock a lot)
• Tirion Fordring (For divine shield deck)
• Ragnaros, Lightlord
• Fandral Staghelm (For Beast Druid)
• Cenarius (For Beast Druid)
• Grommash Hellscream (all purpose warrior finisher?)
• Malkorok
• Archmage Antonidas

Ragnaros would be a great addition to any midrange deck.
Jaraxxus is great if you're playing warlock. Every other legendary is different.
Since you listed Druid, Paladin and Mage as your second choice, if you wanna play any of those, you might need Fandral, Tirion or Antonidas. Altough I'd get Ragnaros, Lightlord before Antonidas.

Yogg is also a good card.

I would look at Jaraxxus and maybe try to make a Control oriented Renolock.

It is good to become familiar with different styles of play because not only will you expand what decks you will be able to play, but by learning the strengths and weaknesses you will also become better at playing your aggressive decks (understanding what type of situations it can be difficult to get out of for a control player, you can try and make those scenarios more often)

I suggest you to craft N'zoth you will be able to play control based decks and you'll have a massive late game threat you can fit it in many decks. Even basic minions(infested tauren, Harvest Golem)will be late game threats plus it might synergise even more with future expansion Have fun!!!!

Priority: 

Warlock - none actually. You can use other charge minion as sub. Leeroy Jenkins is good, but you are missing other better legendary cards; so I won't recommend.  Lord Jaraxxus decks aren't good at the moment. But if you want to choose between those two, I'd go with Lord Jaraxxus.
Hunter - none.
Shaman - none.
Druid - Fandral Staghelm. 
Paladin - craft both Tirion Fordring and Ragnaros, Lightlord.
Mage - Archmage Antonidas.
Warrior - Grommash Hellscream.
Priest - none.
Rogue - Edwin VanCleef

Now for the rest & neutral cards: 

Yogg-Saron, Hope's End - it fits in most decks.
Twin Emperor Vek'lor - must for all C'Thun decks. 

N'Zoth, the Corruptor - if you have good deathrattle cards, you can go for it. 
Ragnaros the Firelord - one of the best legendary card. 

Druid: Cenarius - if you have other Token cards.
Warrior: Malkorok - don't craft till you collect better legendary cards. 

Onyxia - good in Druid decks. Again, not a must have card. 
Alexstrasza - unique, but sadly not much used currently.
Ysera - great card, but doesn't see play like before. Too slow.

Y'Shaarj, Rage Unbound, Deathwing, Deathwing, Dragonlord - none are really worth crafting.

My bad. Fixed it. ^^

John:

I was in the same boat as you not long ago,. I've increased my number of Legendaries in the past couple weeks.

First off, I want to say you should be playing Sylvanas. She's a great card, makes people play sub optimally as they don't know how to deal with her. 

Second, to get even more value out of her, I'd recommend crafting Nzoth and run him and Sylvanas in a Hunter or Rogue deck, as they both have excellent death rattle minions (Savannah Highmane, Infested Wolf for Hunter, Tomb Pillager and Undercity Huckster for Rogue) that can be complemented with many of the neutral death rattle minions.

On another completely different note, if you run C'Thun at all, Twin Emperor Vek'lor is a must. Many of the C'Thun class minions are excellent, and Vek'lor makes a huge impact on any C'Thun deck.

Some other notes:

Ysera is excellent in a war of attrition, but not great against faster decks. Leeroy Jenkins is a ton of fun and works as a finisher in most decks, especially those with buffs.

Craft Dr. Boom.

I would craft N zoth as it allows you to build decks for almost every class even if you don't have that many cards, N zoth is always a good drop when constructing around him.

If you want a more consistent one I think Tyrion is the go to, is always going to be strong and is always going to be on standard play.