The most reliable build when you want to win is probably Phantom Lancer using the splash damage from Orb of Venom/Eye of Skadi in conjunction with his illusions. Completely independent of team composition and solos all bosses. It is pretty boring to play though...
Skills are 1 point in Doppelganger, Max Lance, max Doppelganger, get Ult when possible, then stats, maybe 1 point in Rush. You can jungle to lane with this. Depending on how your other lanes are doing, a few Tower Upgrades (enough to give them split shot) would probably be very helpful too so you don't lose other lanes before you get your core progression down.

Start:

Chicken
Orb of Venom
Your pick (Stick, second orb, stout/queeling, consumables, etc.)

Early:

3x Orb of Venom
Tranquil Boots
Horde Wand

Later:

Eye of Skadi (Rush this)
Heart
Anything you want

you can try Lich with max W to buff tower, and spend all your gold to upgrade tower. if you can kite boss with dagger, I am sure you will last till skeleton king wave. shiva1+shiva2 are really helpful to have. Lich can handle mid by himself really well.

it goes the same to Alchemist , Lvl 1 go for Greed ( make sure you collect all the bounty runes at start , you rush to river to collect 2 and tp back for other 2 , you will probably be able to have tower lvl 3 about 2mins) , max Q for more dmg and armor reduction.

Tower lvl 7-8 is good enough till end game.

Good heroes to beat Ultra with:
- Kunkka rushing Rapier asap.
- Axe rushing Heart of Tarrasque.
- Underlord cus he is insanely OP.
- Drow Ranger with orb of venom, while using her Q at its full power.
- Riki rushing exp to get lvl 6 asap.
- Sniper with maxed out Q and some attack speed items.
- Troll warlord spamming both W spells. Maybe a good thing to buy orb of venom as well(always start with E skill).
- Slark rushing attack speed items.
- Phantom assassin with few Clarities and maxed out Q(getting cleave+orb of venom may be a neat idea, too).
- Shadow Fiend with maxed out coils, then souls+aghs.
- Terrorblade with maxed out Q.
- Windranger with maxed out W.
- Skywrath with maxed out Q and fast mana regen.
- Pugna with nether turrets.
- Invoker with WEE+QWW.
- Batrider with a single orb of venom(You'll love it when you try it).
- Venomancer with maxed out E then W.
- Juggernaut with his Q+W combo(Don't bother yourself with ulti, though).
These are heroes I've tried myself and won the game with for atleast several times.

People also say, you can win with Ogre magi, Jakiro, Necrophos, Enigma, Broodmother, Templar Assassin, but I personally never found them as effective as those above.
There are also few good heroes like Omni or Legion commander, but you should have a decent team to be effective with those guys around.

good communication is key.
thats how I won 2 rounds of ultra from countless of tries.

Windrunner +centaur

pudge + enigma

Originally posted by Pory:

The most reliable build when you want to win is probably Phantom Lancer using the splash damage from Orb of Venom/Eye of Skadi in conjunction with his illusions. Completely independent of team composition and solos all bosses. It is pretty boring to play though...
Skills are 1 point in Doppelganger, Max Lance, max Doppelganger, get Ult when possible, then stats, maybe 1 point in Rush. You can jungle to lane with this. Depending on how your other lanes are doing, a few Tower Upgrades (enough to give them split shot) would probably be very helpful too so you don't lose other lanes before you get your core progression down.

Start:

Chicken
Orb of Venom
Your pick (Stick, second orb, stout/queeling, consumables, etc.)

Early:

3x Orb of Venom
Tranquil Boots
Horde Wand

Later:

Eye of Skadi (Rush this)
Heart
Anything you want

This doesn't work, orbs don't do splash damage on illusions and you have nowhere near the damage to deal with the waves nor the tankiness.

Unless your forgetting something like say, upgrading your tower 6 or 7 times.

Tower kills only grant ~30% of gold you could possibly have. Also, relying on towers too much will only lock you up at the base. You can't bring a tower with you to push, can you?

A pugna can very easily run off and solo towers until he reaches the mega towers, at which point he needs tons of farm to solo those, or a team member to help.