I think UB is in fact the strongest, both by the numbers, and by personal experience.

• He has the Strongest, most efficient stun in the game. Stunning a Demigod for 2 seconds, while ALSO doing damage to them and draining a bit of health is amazing.  It has no casting time, and ticks 2 seconds off of the Cooldown of spit. It is the only instant cast stun in the game (Closest second is boulder roll) and makes him ideal for ganking.
  
• He has the best group ability in the game, in Ooze. Ooze is truly a fantastic skill that scales amazingly well in fights. The damage it deals to opponents is huge, and the damage it negates is also large.
• He has perhaps the strongest direct damage ability, in spit.  It may not be THE best, but it is definitely up there. There are a few ways to counter the effect, though the medicine is sometimes worse than the cure. Even then, the times (most times) when you cannot counter it, it deals very significant damage to the target, in fact it deals the most damage of any ability in the game.

Erebus and Sedna make sense as trailing the leader.  I'd argue that sedna is like the Immovable object, and Unclean beast is the unstoppable force. Erebus though, is the dancer.

• Erebus has the easiest to use minion army.  Just a point or two in skills gives him quite a good army even without a build 'focusing' on minions.
• Erebus still as one of the better nukes in the game in Bite. It is all that *and* a bag of chips, with a direct health swing that rivals Spit, but -speed and -armor too.
• Erebus has, what I think is, the strongest single ability in the game: Bat swarm. The best chase ability AND the best run ability, especially at rank 2. Oh, and it does fire nova damage at the same time.
• Mist represents another extremely flexible ability, as it is a very strong 'counter' to a lot of other abilities.

I would say that Erebus is truly the finesse player in the game right now. He dodges, he swoops, and he has one catch-all nuke to help out in combat. He won't always have the most kills... but he'll get a lot of assists!

Sedna, however, is the Rock, and I feel I shouldn't even need to describe her much.

• Cat-Lady has an entire suite of awesome abilities. The hardest part of playing her is knowing which ones to pick when. Silence, SUPER PRIESTS, a little more pounce, a little more heal... heck even Magnificent presence is very much worth considering.
• High Priests. Yes, they deserve their own bullet.
• Sedna also enjoys the most consistently prolonged early game of any DG.  She is almost impossible to drive from a lane if she buys priests, and picks up healing wind. By level 5, the only reason she'll ever leave combat is to go shopping.

Sedna's only weakness would be that she doesn't do strong damage in 2v2 or 3v3 encounters... except that she has pounce, and can often spam the heck out of it if her priests are doing all her healing. It may not have the kick of spit or Bite, but it is an interrupt and a great way to lock down a runner.

UB

Erebus and Beast have the fastest starting speeds, at 6.3 movement. All things being equal you cannot outrun them.

Add in specced abilities to help them chase down prey, and that is enough of a difference. Plus they are just hella strong.

All they need to do is use their speed and dominate the first few minutes of a match to gain an edge and keep it.

As much as I don't trust the stats at all, this sounds about right from my play experience.

UB's stun also effectively stuns him, just no interrupt to him and he gets the health.  I think with that, and the fact it can be interrupted and such, makes it one of the better-designed ones.

Also, is anyone up for running the numbers through the statistical equations to determine whether the variance in win percentage is significant?

It does not stun him, the beast can still activate abilities while under the grasp, and attack as normal afterwards. I'm doubting that is how the ability is supposed to be used, but that is what happens now.  You can grasp and then use a 2 second potion and be done by the time they are unstunned, or just about.

Pretty sure Minion Erebus and Sedna are the best

QQ more about UB though

Hm, how do they calculate the win percentage they display next to each Demigod?

Number of total games != Wins + Losses.

my top : 1- Erebus Minions Build , 2 - Sedna HP stacking + priests, 3 - UB Hybrid Spit/Ooze build

Nope. If you cancel grasp the Beast stops to drain health, but the enemy is still stunned.

That argument gets made in a lot of games, but it always runs into the same circular problem. Specifically, good players will tend to pick good DGs is because they feel more effective, so its not some random distribution wherein more good players just happen to be playing certain DGs. As such its difficult to determine whether player skill or raw DG power is responsible for the win rates we observe. It is of course, a bit of both, but the extent of the effect of each one, individually, is difficult to ascertain. 

However, it is true that based on sheer numbers the win rates posted in this thread are far more indicative of the power of DGs at an intermediate, pantheon level of play. Since balance is concerned with the maximum power level in the hands of a good player, the win rates are not particularly relevant in observing effectiveness. Although I must say that the win rates do roughly follow what I would expect based on my impressions of high level effectiveness.