Which episode to start watching One Piece from to see Ace and WhiteBeard getting involved? I want to start watching One Piece again but don't want to start from the beginning. I know most of the beginning where they assemble a crew and fight Arlong, Buggy, Crocodile. 

I want to start at a point where all the big badass characters like Ace and WhiteBeard are involved while still knowing what's going on.  Which is the best episode to start from?
It's difficult to give an objective answer, so I'll summarize the arcs to give you an understanding where you should start watching without giving too much spoiler. I'll stick to recurring characters and exclude filler arcs, as I haven't watched those. For the exact episode numbers refer to this page.

If you want to skip to where all the big badass characters like Ace and WhiteBeard are involved you should skip to the Marineford Arc. In that arc you will see an all-out war going down between Whitebeard and the Marines to save Ace. I would recommend you to start at the Sabaody Archipelago Arc though, where a lot of new characters were introduced that will recur in the story. You could skip inbetween arcs if you want. If you would do so, you'd only have missed the introduction of Franky and Brook and the backstory on Robin.

East Blue


  First half of the Straw Hat pirates are introduced.



Romance Dawn Arc:

Introduction of Luffy, Zoro, Coby, Red Hair pirates, (Alvida and Helmeppo).
Luffy rescues Coby and Zoro from the marines.

Orange Town Arc:

Introduction of Nami, Buggy (and his pirates).
Strawhat pirates vs Buggy pirates.

Syrup Village Arc:

Introduction of Usopp, Going Merry, (and Jango).
Strawhat pirates vs Black Cat pirates.

Baratie Arc:

Introduction of Sanji, Mihawk, (Fullbody, Johnny and Yosaku).
Strawhat pirates vs Krieg Pirates.

Arlong Park Arc:

Introduction of Arlong, Hachi, (and Mohmoo).
Backstory on Nami and Strawhat pirates vs Arlong pirates.

Loguetown Arc:

Introduction of Smoker, Tashigi and Dragon.
Luffy will be Pirate King.



Alabasta


  Robin and Chopper join the crew.



Reverse Mountain Arc:

Introduction of Laboon, Crocus and the Rumbar pirates.
Strawhat pirates enter the grand line.

Whiskey Peak Arc:

Introduction of Baroque Works and Vivi.
Zoro vs Baroque Works.

Little Garden Arc:

Introduction of Mr 3 and two giants of Elbaf, Dorry and Brogy.
Strawhat pirates vs Baroque Works.

Drum Island Arc:

Introduction of Chopper.
Strawhat pirates vs king Wapol.

Alabasta Arc:

Introduction of Robin, Mr 2, Mr 1, Crocodile, Ace.
Strawhat pirates vs Baroque Works.



Sky Island


  Introduction of some of those bad-ass characters.



Jaya Arc:

Introduction of Sengoku, Tsuru, Kuma, Doflamingo, Gorosei, Whitebeard pirates, Blackbeard pirates and Noland.
Getting ready to go to Skypiea.

Skypiea Arc:

Introduction of sky islands.
Strawhat pirates vs god Enel and his priests.



Water 7


  Franky joins the crew.



Long Ring Long Land Arc:

Introduction of Aokiji.
Strawhat pirates vs Foxy pirates.

Water 7 Arc:

Introduction of Franky, Sogeking, Thousand Sunny, (Spandam and CP9).
Strawhat pirates vs Franky Family
Luffy vs Usopp
Sanji, Sogeking and Franky vs CP9. 

Enies Lobby Arc:

Backstory on Robin and Strawhats learn new battle techniques.
Strawhat pirates vs CP9.

Post-Enies Lobby Arc:

Introduction of Garp and Dadan.
Ace vs Blackbeard.



Thriller Bark


  Brook joins the crew.



Thriller Bark Arc:

Introduction of Brook, Moriah, (Perona, and Lola).
Strawhat pirates vs warlord Moriah
Strawhat pirates vs Kuma.



Summit War


  Shit goes down.



Sabaody Archipelago Arc:

Introduction of Duval, Kizaru, Rayleigh, the Eleven Supernovas, World Nobels, (Fisher Tiger and his Sun pirates).
Strawhat pirates vs Rosy Life Riders.
Eleven Supernovas vs admiral Kizaru and Pacifista.
Strawhat pirates vs admiral Kizaru.

Amazon Lily Arc:

Introduction of Boa Hancock.
Luffy finds out Ace has been imprisoned by the Marines.

Impel Down Arc:

Introduction of Jinbe, Ivankov and Magellan. 
Luffy brings chaos to Impel Down prison.

Marineford Arc:

Introduction of Akainu, Blackeard's new crewmembers.
All-out-war of Whitebeard pirates and their allies vs the marines and the shichibukai in order to save Ace from execution.

Post-War Arc:

Introduction of Sabo and Haki.
Backstory on Luffy, Ace and Sabo.
Three year training-timeskip of the Strawhat pirates.



Fishman Island


  A glimps of the new techniques the Strawhats learned during the timeskip.



Return to Sabaody Arc:

Strawhat pirates regroup after their training.

Fishman Island Arc:

Introduction of Shirahoshi, Big Mom and Koala. 
Strawhat pirates vs New Fishman Pirates.
Backstory on the Sun pirates.



Pirate Alliance


  Doflamingo doing a lot of sick shit.



Punk Hazard Arc:


Introduction of Dr. Vegapunk, Caesar Clown, Kaido, Kin'emon and Momonosuke.
Strawhat pirates and Heart pirates form an alliance.
Strawhat pirates and Law vs Caesar and the Donquixote pirates.

Dressrosa Arc (ongoing):


Introduction of Tontatta, CP0, Fujitora, Bartolomeo, and A LOT of other characters that might make a recurrence in future arcs.
Strawhat pirates vs Donquixote pirates.