With Tarkir: Dragonstorm being a heavily multicolor set, it perhaps won't surprise you to know that many of the best rares and mythics are also gold. Indeed, this list is going to look very similar to another recent article I wrote. There are a handful of exceptions, though, so let's dive right into the best rares and mythics in TDM limited.
Dragonback Assault
What can I say about this card that hasn't already been said? It's a sweeper that also takes over the game on its own. It requires a little bit of finesse in terms of building your mana base correctly, but with only one pip of each color, that's not too tricky. It stabilizes the board, then it quickly puts you in the driving seat by making a 4/4 flier (almost) every turn. What do you have to do to get them? Just hit your land drops. This card asks very little of you and it gives so much back that it's easy to see why it makes both lists.
Marang River Regent
Standard all-star, Marang River Regent is the only mono-coloured card on the list (though Elspeth, Storm Slayer only narrowly missed out). Both halves of this card are good in the right spot, and the fact that you get to choose the one you need when you need it is the cherry on top. You should typically look to cast the dragon, I think, but it's nice to have the option to draw three if you need it. A three-for-one on-board is better than two cards in the hand, as the saying goes. The fact that this card shuffles back into your deck after casting the spell side makes for a nice combo with itself, too. It helps to make sure you get the lands you need to cast the Dragon by the time you re-draw it. Being standard playable doesn't always translate to being good in limited, but it certainly does in this case.
Ureni, the Song Unending
Another card that appeared on the best gold cards and another card where it's easy to see why. If you can get to eight mana and still be even remotely in the game, this card makes it extremely hard to lose. If you're against a Black or White deck, yeah... I suspect that the reason this card's game-in-hand win rate isn't even higher is because you can die with an eight-drop in your hand, even in this format.
Ugin, Eye of the Storms
Now this is an interesting one. Being colorless and going in any deck is obviously helpful. I had this in a Boros aggro deck, where it is probably at its weakest, and I still trophied thanks to how absurd this card is. What's really funny, though is that it's still so good despite really only having one ability. That ability is incredible, though, and it sets Ugin to nine loyalty, which is a hell of a lot for your opponent to try and attack. Ugin also removes your opponent's most important permanent even if he doesn't resole, so that helps. Yeah, three colorless mana isn't doing much when you're already on seven and the -11 might as well not exist in limited, but the card advantage and life total bump provided by his +2 is back-breaking.
Honorable Mention: Sinkhole Surveyor
I'm throwing the bird in here for two reasons. Firstly, if this card is in your opening hand, it's actually the fourth best rare in the set with a 65% win rate. Secondly, if you draw it at any other time, it plummets to 55.5% - going from A+ to C- on 17lands. It's an incredible snowball card if you can get it going early, but it's a sad chump blocker later on.
Jeskai Revelation
Here we go again; the third and final card that crosses over from the best gold cards list. I got to humble brag about my Ugin deck just now, so it's only fair that I also say I have contributed to the 31% of games where players with this card lose. Still, that leaves over two thirds of games where this card is drawn resulting in a win for that player. It just does everything. I mean, have you read that text box? Admittedly, it might take you a while, but it's worth the time. It's one of the best comeback cards ever printed, removing two creatures, providing two blockers and four life while also drawing you into more answers or threats. Oh, but if you're even or ahead, this ends the game real quick. As a seven-mana card with just one pip of each color, it is laughably easy to cast for such a game-warping bomb.