Maybe you've been around since the beginning, you have every character in spades, you have some of them in hearts and diamonds and clubs, and this post is not for you.
Maybe you're stubborn, and you like units as units are, and you don't want to touch them. This post is not for you.
If you're new-ish (and even if you're not), chances are you have to move Talis around to be successful in the gauntlet. And at some point you may find you've exhausted all the toons with which you are familiar, and the gauntlet is no longer about toons at all. It is 100% about talis. It stands to reason that you want go-tos so you're not foundering around and giving yourself a headache. Maybe you even want to know what another player thinks is good.
Now we're getting somewhere!
I chose to focus on 5-sets in this post for a couple of reasons. One, 5-sets are a little simpler than 2-tali combos (2 and 3 sets, say), making them more ubiquitous between toons, whilst being easier to equip than 6-sets (because not all color slots need to be the same). Second, they are easier to move between units (along with your favorite cosmics) without losing track of where you put everything. And third, I am (personally) missing some of the most explosive and valuable 6-sets, making them a touch trickier to rate. Narrowing the scope to 5-sets makes evaluation a lot easier than trying to consider the sum-total of talis in a quick post you're probably not still reading, anway.
Without further ado, let's begin the march backwards.
5) Alchemy. Nothing can ruin your day in Gauntlet like a debuff you weren't expecting, because you know you're probably not going to go first. Though Alchemy is perhaps not necessary on your Alex Ed., Moonchild, Musashi Arena team, it's a wonderful Tali to find a spot for in Gauntlet so you don't lose for dumb reasons. The Alchemist is a trick that only works once, but Alchemy is the gift that keeps on giving.
Tali Comparison: Celestial. The comparison isn't perfect, but probably you're not going to find (or want to find) room for both on your Gauntlet team, and because of the slots in common (Magus) I've had to make this decision more than once. Celestial obviously has better first-turn damage mitigation than Alchemy + some damage reduction for future turns, but consider that invincibility sometimes works just as well as Celestial's first perk and the next tali on our list does better for its second (effects don't stack).
Give me Alchemy over Celestial in Gauntlet 8/10.
4) Ragnarock. It seems blasphemy to rate a 5-set this devastating in story mode this low, but them's my feelings. Don't count on crippling your opponent's team in Gauntlet with Ragnarock alone, but the team damage reduction alone makes this 5-set a fantastic (and toon-independent) support Tali if you're wanting for a spot. Ragnarock is so good (everywhere) because it plays offense and defense at the same time. Opponent planning to one-shot you with that G. Minotaur? Ha ha ha!
Tali Comparison: Vipers. To be fair, I don't own Vipers and I'm not entirely sure what they do. However, Ragnarock gives you all the goods up front and protects your whole team better than Viper, which seems to be what you want more often in a format in which you might otherwise get popped before you have a chance to play.
Give me Ragnarock over Vipers in Gauntlet 10/10 (because I don't own Vipers).
3) Artifact. Artifact is my favorite whole-team heal Tali in the game. It doesn't offer any damage reduction, but this is actually to its advantage because it means that it (the Tali) can offer you better absolute healing potential and a whole lot of other good stuff (some fury, if that's relevant to you, and some auto-cleansing when you get attacked). Not to mention, the lack of damage reduction doesn't matter one hoot if you are able to combine Artifacts with another tali (cough, *ragnarock*, cough) that does the job for you. Artifact makes it this high on my Gauntlet list precisely due to its synergy as a set with Ragnarock and another Tali lower down.
Tali Comparison: Ying/Yang. There is really no comparison with most of the other Sentinel 5-sets, which are pretty mediocre, but Ying/Yang is an honorable 5-set because it seems to fit on a lot more units than any Assassin-or-Gunner 6-set alone, and it makes for a good comparison due to its team healing. Ying/Yang protects a single unit a little better than Artifact and is an OK stand-in, but its absolute team healing potential is less than Artifact and the damage reduction once again doesn't matter if you've already but a Ragnarock-wielder on your squad.
Give me Artifact over Ying/Yang in Gauntlet 9/10... so long as you pack support.
2) Forge. Expecting something else? There are plenty of units that apply perma- or perfect death or some variant of same, but with forge you don't even have to think about tracking them down. That's the offensive bit. Did I mention that a lot of gauntlet teams have no reliable ways to remove golden buffs, and so Forge can literally single-handedly win you the match?
Forge was absolutely transformative to my PvP experience when it first became a thing, and in the time since its release nothing has really changed. Half the time I conceive of a fun and unique defense team, I have to scrap it in short order because Forge sets give it no recourse and no path to victory. Forge is the Tali set you want to find a slot for if you ignore the rest of this post because it plays offense and defense so well (simultaneously).
Tali comparison: Labs. Yes, I went there. Why not? Labs can cover your beatstick effectively while inflicting Stop, which is a solid combo for sure. They probably have more potential in Arena and on certain toons. But in the Gauntlet, when toons don't matter and Talis do, Labs protect a dude and Forge protects your team (while making sure the other team does not revive). Because of the Lab/Moonchild debacle, the Dev. team has begun to supply players with more an more answers to Labs, even on a solid toon. The same is not true for Forge.
Give me Forge over Labs in gauntlet 10/10.
1) Yggdrasil. This may be a bold and premature call, but if my recent Arena experience is any indication, I am not letting any cats out of bags by announcing, "This tali is good!" I have always liked Block-ers for the Gauntlet, because typically when you're scraping the dregs of your inventory you want to make sure the hammer-toon on your team doesn't die on turn 1. Yggdrasil makes garbage toons awesome blocking units... and not only that, it makes them awesome blocking units that can't be PDed with built-in damage reduction. Artifact can be paired with Yggdrasil (instead of Ragnarock) if you want to take a Super Turtle Offense approach to Gauntlet (see my Moonchild post). And? It's the best Tali that has been released since the release of Moonchild with built-in Moonchild hate (and as such, helps you take down some sound defenses... though you still need to find a way to cleanse that Doom). All of these things, in tandem, elevate Yggdrasil to the top of my Gauntlet tali rankings list.
Tali Comparison: None.
Give me Yggdrasil over None in gauntlet 10/10.
***
Notable snubs: you may have noticed no Gunner 5-sets appear on my list. IMO Valhalla and Light both suffer from the same flaw in Gauntlet, which in a word is Forge. You simply can't rely on regeneration effects v. a lot of common defense set-ups, capping the upside of these Talis. In addition, Light seems to only come Bulwark for me, and in all of my experience using it my toons seem to get one-shot before a single heal effect is triggered. You could try pairing it with Ragnarock, but then I think Light begins to compete with space with Artifact, and Artifact just seems better.