It might not be the best card in the set, but one of my favorite cards in Khans of Tarkir is [scryfall]Pine Walker[/scryfall].
Why, you may understandably ask? To me its the subtlety of the decisions regarding the Pine Walker, Qal Sisma Ranger (and yes, that was an awkward and a stupid joke – so feel free to use it).
Generally, there are two kinds of morphs: Those you can use to fill your curve on three and then flip face up with a cost lower than their actual cost and those you can play for their normal cost to act like fairly vanilla creatures, but you have the option to pay more to get some nice effect.
The first kind is more common:
You still pay more total for the creatures if you play them face down and then flip them, but since they usually cost six to turn face up and you often won’t be at six mana on turn six, you can probably still get these guys on the table easier if you use their morph ability.
The second kind is often much more fun for the one who plays them:
I generally wouldn’t play Spellsnatcher face up, but its an option. It is a rare, so I haven’t faced the decision that often. The others… depends on the situation. They do have one thing in common: You’d very much like to manifest them, because its cheaper to turn them face up that way and you still get the effect. Your opponents aren’t going to be expecting anything this bad either, so the blowout is even bigger.
But then there’s the Walker.
Its Morph cost is pretty much the same as its casting cost, so if you have something else to play on turns three and four, you don’t need to morph it. You can usually just cast it as it is. But, the one turn pseudo-Vigilance provided by the ability is also nice. It can be a big surprise on your opponent, who might have thought the way is open for their attack.
The nice part is that the decision is not easy. Do you need to play it face up now, so that you can attack with and untap your face down [scryfall]Woolly Loxodon[/scryfall] next turn? (One of my favorite plays, BTW.) Sure, the other Morphs force you to make decisions as well, but with the Walker they always seem to be much closer than with the others. With the others, you just want to play them on curve or if you have another play on curve (such as with [scryfall]Icefeather Aven[/scryfall]) or a chance to blow out your opponent, you choose that. With Walker you don’t always know the best way to approach the situation.
The usual considerations of course apply as well (opponent possibly holding a [scryfall]Disdainful Stroke[/scryfall] or a [scryfall]Debilitating Injury[/scryfall] for example). Of course, a 5/5 for five is in itself a great card in limited, even if you discount everything else, so pick it if you can. It passes the vanilla test with flying colors.