It's the gold one. That's the one I use too, and the short ears look better than the large floppy ones

get the Deadly cloak too. If you're hit while cloaking it's usually by a bullet so stunning nearby enemies isn't that great.
Plus you can easily use the "flash charge" glitch where it does a second charge attack if you are doing one while it wears off, and the first attack already does boosted damage.

it doesn't matter much which quills you'll pick. Vengeful is the gold ones. However, virulent works better when the monster stacks together like in vortex. Also virulent's poison is slightly stronger than original quills. Sure, vengeful is better at general situation. But, the quills characteristic makes it hard to optimize the damage (only 4 quills per monster, gone just few seconds that you should attack fast, and vengeful quills only goes to nearest monster which lack of spreading poison ability unlike virulent).

That being said, just pick whatever quills you want.

Also try searching Google for something like "sprite ultimate site:forums.spiralknights.com".

Vengeful has the potential of more damage in general. However, unless you bring a weapon with multiple attacks at once (like dark retribution, autogun, brandish charge, cutter) and correct timing/position to unleash your attacks, quills damage is close to none, but only poison status.

Quills stick to enemies that they come in contact with, and explode when the enemy is attacked by another source. They do not damage on impact, rather, only poison on impact. There is a limit as to how many quills can be on a target, however (about 4 or 5). Quills last only a few seconds, as well.

Because of the limit of quills on a particular target, some quills can be wasted by landing on an enemy that is already "full" on quills. Not attacking enemies that have quills will cause the potential damage to disappear, as well. This leaves you with basically no damage from the highest-dps Sprite attack there is.

Vengeful quills makes it so any original quills that explode on a target release a secondary quill, that does not produce any more quills. Both quills deal damage, making it damage much more than the other Ultimate. Note that the secondary quills have to jump to another target; they cannot go back to the original target.

if you use the sprite to do more damage, do what the bear said.
if you use the sprite to take less damage, do the other choices.
that being said, defense is way worse than offense in this game.

"Choose the other ones if you would not like to do more damage."
not necessarily true on all situation, it could yield more damage overall by taking advantage of AoE poison status.

poopsie, I think you should reconsider. poison doesn't quite do the damage that the quills can get, and the AoE effect of virulent isn't that great from what I've seen.

For Maskeraith and with what I know:
1 - Vengeful Quills - can deal twice the damage and inflict poison to more targets, you just need to hit twice the quills. Virulent Quills is said to have a more potent poison proc, but it doesn't bring any more utility than it already has and, for what I've seen, the explosion is insignificantly small.
2 - Deadly Shadow Cloak - amazing for burst damage, can be difficult to use on temporarily vulnerable bosses but can be well rewarding. Don't know about the Vengeful Cloak but it seems useless, specially when hit by a bullet, distant from anything.
3 - Chaotic Haze - It's not much great, but it's much better than the whopping 8 damage the haunted haze's missiles can do, and you don't have to deal with the timing hassle either.

And while at at,
Drakon:
1 - Meteoric Fireball - it seems to be the most reliable, since it can hit multiple targets more easily, giving a better chance setting someone on fire. If the chance of proc'ing status effect was guaranteed, I would have opted out for Concussive for the stun. Damage is not a thing you should consider since it scales awfully, unless OOO improves it sometime in the (distant) future.
2 - Scorching Barrier - It actually makes the skill good for bruising, specially if you run around an enemy counter-clock wise (if not mistaken). Backfire is good too if you like to go fast... for 2 or 3 seconds. After that, it's just like using the skill in its normal rubbish mode.
3 - Explosive Firestorm/Frenzied Firestorm - The former not only allows the skill to actually inflict damage (quite effective against linchen colonies!), but it also can stagger weaker mobs (can even tumble elite gremlins!). The latter gives an attack damage and speed boost to all party members. Duration does not stack, unfortunately, but the bit extra DPS it provides can still help for a short moment on crowded rooms. Pick if you want the skill to be either mostly offensive or supportive.

Seraphynx:
1 - Disintegration Ray - Pretty obvious choice: Not only it can deal more damage than the other choice in almost every condition, but it also lets you and other teammates deal more damage too!
2 - Iron Heart Attack - See Makeraith's 3, without the timing part.
3 - Valkyrian Aura - Maxed CTR on-the-go is much more useful to you and your team mates than having your shields regenerating quicker, specially if you don't want to use Chaos set due to its status weaknesses, but you want the supreme CTR buff it offers. Plus, everyone has a dash skill that you can use to dodge ANYTHING (except traps) and get you to a safe spot where your shield can recharge safely. Not to mention your pet gets a fluffy tail!

I feel like I should have written so much, but I wanted to justify my choices. Got nothing better to do anyways.

Okay say... the most ideal situation, you charge a vortex, quills the zombie right at the center of vortex, so 7 zombies got suck into vortex got poisoned right after quills exploded, you charge voltedge to trigger it, then combo. 7 poisoned zombies in vortex got initial slash (not all poisoned yet), 2 - 3 explosions, first swing and final swing (the rest of these attacks got poison advantage). Now, compare to vengeful quills which affect up to 2 - 3 monsters mostly. I could bet damage is comparable between those two situations. Also, it's easier to knockdown monster mobs when all poisoned which makes it safer for you to go wild on vortexed monsters.

Of course, this is just me talking silly. And all of that kinda meh rarely happened.
Anyway, back to my first intention, if someone asks me which quills, I'd answer pick whatever you like.

It's just me being hardheaded. Sorry for making you confused ;p
(though part of me love to answer in such unclear manner because of your silliness)

poopsie, you would need quite a lot of monsters and a vortex to make that work. there are very few instances in which you can trap more than 4-5 zombies.

overall Vengeful is better, until you have more than 5-6 monsters in a vortex.