There are many special groups of Digimon, which are more-or-less universal in all Digital Worlds.
The Holy Beasts

Qinglongmon is the Guardian of the Eastern part of the Digital World. He's Final Level, with a Data attribute. He's a Holy Dragon Digimon, and also is a member of the Four Great Dragons. His attacks are Blue Lightening and Wind Electricity.

Baihumon is the Guardian of the Western part of the Digital World. He's said to be the strongest of the Guardians. He is Final Level, with a Data attribute. He's a Holy Beast Digimon. His attacks are Iron Claw and Vajra.

Zhuqiaomon is the Guardian of the Southern part of the Digital World. He dislikes humans. He is Final Level, with a Virus attribute. He's a Holy Bird Digimon. His attacks are Crimson Blaze and Purgatory Claws.

Xuanwumon is the Guardian of the Northern part of the Digital World, and is the oldest of the Guardians. He is Final Level, with a Vaccine attribute. He's a Holy Beast Digimon, with the attacks Phantom Mist and Black Hail.

Huanglongmon is the Guardian of the Center, and may be the leader of the Holy Beasts. He is Final Level, with a Data attribute. He's a God Beast Digimon, with the attacks Tai Chi and Oukai (Which translates to Yellow Circle). Huanglongmon is not the God of the Digital World as some previously thought.
The Great Angels
Because three of the Great Angels were important characters in Frontier, descriptions of what they did there are listed here...

Seraphimon -
Seraphimon is the highest ranked Angel Digimon aside from Lucemon. He, Cherubimon, and Ophanimon act as the Guardians of the Frontier Digital World. Seraphimon is Final Level, with a Vaccine attribute. He's a Seraphim Digimon with Seven Heavens and Testament for attacks.
When Cherubimon revolted, he first attacked Seraphimon. He sent a massive army to the Forest Terminal, Seraphimon's residing place, and hit Seraphimon with Lightening Spear. Seraphimon was badly injured, but Ophanimon saved him from Cherubimon. Later, the Chosen Children Ophanimon had called to the Digital World freed him from the crystal Cherubimon placed him in, but he was killed as a short time later. Izumi grabbed his Digi-Tama, which Bokomon took and put in his belt. His egg allowed Takuya and Kouji to Double Spirit Evolve, and it later hatched into a Patamon. Patamon does not remember being Seraphimon.
Later in the series, the spirit of Seraphimon temporarily emerged from Patamon to give some words of encouragement to the Chosen Children in their darkest moment, as did Cherubimon, Ophanimon, and the Legendary Warriors.

Cherubimon -
Cherubimon, along with Seraphimon and Ophanimon helped to guard the Digital World. He is a Final level, and is a Vaccine. He's also a Cherubim Digimon whose attacks are Heaven's Judgement, Lightning Spear, Thousand Spear, and Lightning Judgement.
Cherubimon was frustrated at Seraphimon and Ophanimon, believing they could not understand Beast Digimon such as himself. Seraphimon allowed him to talk all he wished, but Cherubimon only grew more and more frustrated. The last time Seraphimon, Ophanimon, and Cherubimon talked together, Cherubimon ran away crying. He hid in a tree, and became corrupted by Lucemon, transforming him into a Virus. He attacked the Forest Terminal, and injured Seraphimon. He captured Ophanimon, and used the Spirits of Wood, Water, Darkness, Ground, and Steel to create Warriors to gather data for him. He captured a human to use the Spirits of Darkness. However, although Cherubimon destroyed much of the Digital World, one by one his servants fell. All the Spirits and the child he used and corrupted fell into the hands of the Chosen. When the Chosen came to rescue Ophanimon, he temporarily regained all but the Spirits of Water and Wind, but Ophanimon burst from her prison, and returned the Spirits to the Chosen. She saw them to safety, and nearly killed Cherubimon with Sefirot Crystal, but Cherubimon escaped, and she died. Cherubimon was later defeated by Kaiser Greymon.
Ophanimon's egg hatched on the yellow moon into Plotmon. Like Patamon, Plotmon did not remember her past life. Later on, Ophanimon's spirit emerged from Plotmon to give the Chosen Children some words of encouragement in their darkest moment, as did Seraphimon, Cherubimon, and the Legendary Warriors.

Ophanimon -
Ophanimon also helped to guard the Digital World. She is Final level, and a Vaccine. She's a Throne Digmon with Sefirot Crystal and Eden's Javelin for attacks.
Ophanimon saved Seraphimon from Cherubimon's wrath when Cherubimon revolted, although she had to be captured. She tricked Cherubimon into thinking that she was working with him, and called hundreds of people into the Digital World. After choosing the Chosen Children for the Spirits, she conversed with them through the D-Scanners, and sent the remaing people back to the Real World. However, four decided to stay a little while and have fun, and so Ophanimon sent an Angemon to protect them. Eventually, the Chosen Children arrived to rescue her, but Cherubimon defeated all of them and took their D-Scanners, along with the Spirits...except for Shutumon. Shutumon held out for a while, but Cherubimon started to use Lightning Spear, and Ophanimon broke out of the Cage of Light to save her. Ophanimon used Eden's Javelin and temporarily uncorrupted Cherubimon, who was in great pain. Cherubimon reverted back to Virus, and was angry at Ophanimon - but she threw away her weapons, and asked Cherubimon to come closer. She turned him to Vaccine momentarily, and took the D-Scanners. Cherubimon reverted again to Virus and woke up, and to his great anger found he didn't have the D-Scanners. Ophanimon gave them to the Chosen, who were transported away. Ophanimon and Cherubimon both attacked and the resulting explosion killed Ophanimon - but Cherubimon was not in great shape, and Ophanimon had bought time for the Chosen to Hyper Spirit Evolve.
Cherubimon's egg hatched on the yellow moon into Lopmon. Like Patamon and Plotmon, he did not remember his past life. Later on, Cherubimon's spirit emerged from Lopmon to give the Chosen Children some words of encouragement in their darkest hour, as did Seraphimon, Ophanimon, and the Legendary Warriors.
It is said that there are seven Great Angels; however the others are not official.
The Seven Great Demon Lords
Lucemon Falldown Mode
Demon
Lilithmon
Leviamon
Beezlebumon
Barbamon
The last of the Seven Great Demon Lords is unknown, but he appears to be their leader.