5th Gen Best Pokémon for defeating the Elite 4 in PKMN Black

A year or two ago when I last attempted to beat the Elite 4, I never managed to win.

Here is my team and their moves.

1. Serperior (Lv. 57)
* Wrap
* Leaf Tornado
* Giga Drain
* Wring Out

2. Seismitoad (Lv. 55)
* Growl
* Attract
* Muddy Water
* Surf

3. Vullaby (Lv. 37)
* Defog
* Faint Attack
* Flatter
* Punishment

4. Unfezant (Lv. 56)
* Air Cutter
* Fly
* Detect
* Growl

5. Scolipede (Lv. 56)
* Venoshock
* Screech
* Poison Tail
* Toxic

6. Stoutland (Lv. 55)
* Leer
* Roar
* Strength
* Reversal

The team is all-male.

Are there any good PKMN and move recommendations for a best team for defeating Elite 4 in PKMN Black?

UPDATE: I gave Surf to Seismitoad and he forgotten BubbleBeam so I could give Surf to him and I am putting a Vullaby in my team to take place of my Simipour.

It's good that you put Vullaby/Mandibuzz on the team though, a Dark type is very good to have against the E4 since two of the four use a type that's weak to Dark. It's not a hard hitter though, so if it doesn't work out for you, you fortunately have plenty of other choices for Dark types. Could also potentially swap out Unfezant for something else if you stick with the Dark/Flying bird.

Having better movesets will help you out a lot too.

Serperior: Wrap was only good in Gen 1 really, here it's a terribly weak move with no significant benefits. Serperior's movepool is not great for typical Gen V runs, so your options are pretty limited, though you could try stuff like Coil or Taunt, or Hidden Power if it gets a good type for it.

Seismitoad: Would be best to replace Growl and Attract. There's better ways to cut Attack than Growl, and Attract is unreliable. Seismitoad gets a good variety of stuff including Earthquake, Scald, Drain Punch (and other Fighting attacks), and some Poison attacks, all of which are worth considering. Would recommend Earthquake especially.

Scolipede: With Toxic, you have a reliable way to inflict poison to boost Venoshock and so Poison Jab isn't needed. Don't really need more than one Poison attack on it, Poison makes for a better STAB from Gen 6 onward. This frees up a slot for Scolipede's other STAB, Bug, which will allow it to act as back-up for your Dark type against Caitlin as well as threaten Grimsley's team. Steamroller and Megahorn would be the ones to look at.

I replaced my Serperior's Wrap with Hidden Power and my Seismitoad's Growl with Scald.

Anything else? Any good moves for my Vullaby?

Stoutland gets the 3 elemental fangs and Crunch, so teaching it 1 or 2 of those over Leer/Roar would be helpful. Vullaby I'm honestly not sure what to suggest to you for it.

What Hidden Power type does your Serperior get btw?