The only top tier decks Sylvanas is consistently being used in are the N'zoth decks, and Paladin is the only class really of note there. This is not to say that she won't occasionally find her way into other deck types, but it's clear that her role inside decks is far from universal. When she is included she's a supportive fill card for slow decks expecting to play other Legendaires. Crafting her early on seems like a mistake.
Will this be true 1-2 years out, though? What are the real choices at this point in time for classic craft legendaries?
Cairne: Sylvanas' brother that disappeared after Naxx and only recently came back
Alexstrasza: Relegated to Freeze mages for the time being and it wasn't that common outside of Control Warrior either, I'm also not expecting Dragon decks to come back for a while
Archmage Antonidas: Mage exclusive, and neither Freeze nor Tempo run it anymore
Baron Geddon: Always a niche Control Warrior option
Thalnos: Miracle rogue
Captain Greenskin: lol
Cenarius: lol
Deathwing: lol (because it's seriously being played now)
Edwin: Rogue exclusive and I don't think ferty is thinking of Miracle
Grommash: Probably the second best control class legendary craft after Tirion, but Control Warrior has been a more consistently viable archetype than Control Paladin
Harrison: Tech option
Hogger: lol
Illidan: fucking lol
King Krush: Even if my control hunter prediction comes true this is still not worth a craft
King Mukla: lol
Leeroy: Not a control card
Lord Jaraxxus: A viable craft, I guess, but handlock is dead right now and Reno will eventually rotate in 1.5 years?
Lolwalker Cho: <--
Malygos: Combo card
Nat Pagle: Maybe if you traveled back in time to closed beta
Nozdormu: lol
Onyxia: lol
Velen: lol
Ragnaros: Ragnaros has phased in and out depending on how fast the meta has been, it's still a good craft in general but I've just gotten more mileage out of Sylvanas than Rag
Syl herself: Nuff said
The Beast: lol
The Black Knight: Tech card that was getting hyped up early this expansion but pretty much disappeared
Tinkmaster: Time travel craft along with Nat
Tirion Fording: Good card
Ysera: A legendary craft that's even slower than Ragnaros and gets into less decks because of that
Narrow that down and what are you left with?
Harrison
Sylvanas
Ragnaros
Tirion
Grommash
Cairne
Alexstrasza
Jaraxxus
Of those, Sylvanas is the obvious winner unless you want to commit to a class.
Let's extend crafts to sets facing rotation. That means TGT and WOTG. It's a rather short list.
~1 year left (TGT):
Justicar: Good craft but again, 1 year left to play it and that's not something I'd recommend first
Varian: Only tempo runs this
~2 years left (WOTG):
Velkor: C'thun exclusive
N'zoth: Nice card, but the thing is you actually need Cairne, Sylvanas and Tirion to make it worthwhile
Yogg'saron: Miracle/Tempo-mage Hail Mary tech
Staghelm: Not a control deck
Huhuran: Eh
Lightlord: Tirion is a better craft than this if you want to commit to Paladin
Xaril: Miracle/tempo exclusive
Hallazeal: Control shaman is yet unproven
Cho'gall: Jaraxxus is a strictly better craft
Malkorok: Tempo card
If you can swallow crafting a card you can only use for a limited time, Justicar is the only option. Between Justicar, Ragnaros and Sylvanas, Sylvanas still seems like the easy choice. Not only is it frequently good for Control, but it also finds its way into Midrange decks, turns on N'zoth and deathrattle synergies, and so on and so forth. Also Sylvanas is my waifu I might be biased???