There are many special groups of Digimon, which are more-or-less universal in all Digital Worlds. You can view a list of the groups and their members on the Notes page in the encyclopedia.
The Four Holy Beasts
The Four Holy Beasts are the guardians of the Digital World in the Digimon Adventure universe and in the Digimon Tamers universe. They each possess twelve Digi-Cores, which presumably give them enormous power. Qinglongmon is the guardian of the east, Baihumon the north, Zhuqiaomon the south, and Xuanwumon the north. Also, it seems as though Zhuqiaomon tend to dislike humans, while Qinglongmon are wiser. Baihumon is said to be the youngest of the Holy Beasts, and Xuanwumon the oldest.
In Digimon Adventure, the Four Holy Beasts were apparently the principal leaders of the powers of light. They orchestrated the creation of the Chosen Children, Digivices, and Crests, but the Dark Masters intervened with those plans, and eventually succeded in defeating the Holy Beasts and sealing them away, so that they could change the Digital World as they pleased. When the Chosen Children defeated Apocalymon, the Four Holy Beasts had Gennai summon the Chosen Children to release their Crests, thereby unsealing the Holy Beasts. However, their plan was not entirely successful, because the Digimon Kaiser had established Dark Towers that kept them sealed away.
However, the the short time between their unsealing and resealing, they managed to orchestrate the D-1 Tournament, in which they summoned dozens of Chosen Children to participate in a tourament designed for the explicit purpose of training Ryo, although Ryo had not been told that. As they knew they stood no chance against Millenniumon themselves, they had summoned Ryo and trained him so that his mysterious power could eradicate the threat Millenniumon posed. This too was not entirely successful, because Millenniumon ripped a hole in space-time and evolved into Zeed Millenniumon, and challenged the creators of the Digital Worlds themselves.
The only Holy Beast that appeared in the Digimon Adventure Zero Two show was Qinglongmon.
In Tamers, the Four Holy Beasts were the ones that had sealed the light of evolution into Culumon to prevent the return of the D-Reaper. That having failed, they managed to unite with the Tamers against the D-Reaper, despite Zhuqiaomon's malevolence toward the humans. They also brought down the Shinigami Reaper.
Huanglongmon is the Guardian of the Center, and may be the leader of the Holy Beasts. He is Ultimate/Final Level, with a Data attribute. He's a God Beast Digimon, with the attacks Tai Chi and Oukai (Which translates to Yellow Circle). Although it was at first believed Huanglongmon was the god of the Digital World, this seems unlikely.
The Three Great Angels
Because three of the Great Angels were important characters in Frontier, descriptions of what they did there are listed here...
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, 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 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.