This won't be perfect and will probably be a mess, but I will try my best.
Sith list:
1.Vitiate
Canon: Most powerful Dark Sider ever, nuff said.-SWTOR, SWTOR:E, SWTOR: Revan
2.Exar Kun
Canon: Once the most powerful Ancient Sith, Unprecedented power curve, Master sorcerer and dark sider, all time great duelist.-TOTJ, TCSWE
3.Dread Masters.
Canon: Most powerful sorcerers in the Empire, second-only-to and hand-picked by the Emperor, centuries of knowledge, Mastered Phobis Devices, Master Force techniques. Held together a Harrower-class dreadnought with telekinesis alone, has displayed Fold Space mastery.-SWTOR, SWTOR:E
4.Darth Jadus.
Canon: Second most powerful Sith Lord in the Empire, during Cold War, Master Force techniques, held together a Harrower-class dreadnought with telekinesis.- SWTOR
5.Darth Traya.
Canon: No outright statement besides being > Meetra Surik, Unprecedented knowledge of Force techniques, some of the greatest feats of farsight ever displayed.-TCSWE, KOTOR:CG
6.Emperor Malgus.
Canon: Achieved state of oneness with the Dark Side(unlocked Force Maelstrom), Sidious praises him as a Sith Warrior, Vader learns from him.-SWTOR😄, BoS.
7.Tulak Hord.
Canon: Took entire systems, broke a Jedi siege, master duelist-KotOR2:TSL, SWTOR.
8.Marka Ragnos.
Canon: Master of mass mental domination, remained Dark Lord over a century.-BoS
9.Darth Marr.
Canon: Unofficial leader of the Empire during 2nd GGW, Quelled Republic/Jedi army on Korriban, Held DC seat for decades-SWTOR, SWTOR:E.
10.Emperor's Wrath
Canon: Defeated Darth Baras & Sel Makor as well as powerful Jedi Masters and Darths, Hand-picked by the Emperor himself to be the Wrath, Terrifies many DC members-SWTOR, SWTOR:A
Honourable mentions: Darth Malak, Darth Nox, Ulic Qel-Droma, Lord Scourge.
Jedi List:
This one is going to be much harder as many Jedi are shown to be extremely powerful in merely one comic and lack expansion afterwards.
1.Nomi Sunrider
Canon: Unprecedented talent in Battle Meditation, Sever Force and Wall of Light. Goes on to be an extremely powerful leader of the Order, Has incredible natural power in the Force, considered one of the all time greats.-TOTJ, TOTJ:C, TOTJ😖, TCSWE, TNEC, POTJ & More.
2.Meetra Surik
Canon: Destroyed the Sith Triumvirate, Revived the Jedi Order. Defeated Darth Nihilus, Trained six apprentices simultaneously. Gained enlightenment, unnatural strength in the Force, praised by Revan, Kavar and Traya. Defeated Traya, Sion and the entire Trayus Academy single-handedly on Malachor V. Rescued Revan from captivity, defeated multiple IGs, imbued Revan with Force energy for three centuries as a Force Ghost.-KotOR2:TSL, KotOR:CG, TCSWE.
3.Hero of Tython
Canon: Champion of the Jedi Order, Defeated Darth Angral and nearly all of the Emperor's sphere of power, including the Emperor, Stated by GM Shan to be greater than herself.-SWTOR, SWTOR:A
4.Barsen'thor
Canon: One of the greatest Force prodigies in the history of the Jedi Order, a natural connection to the Force granted power at four years old that even powerful Jedi would have gained by fifteen. Defeated half a dozen children of the Emperor and then the First Son himself, the powerful Syo Bakarn. Unprecedented innate knowledge of the Force gained them the title they are famous for. Brought together entire alien armies to support the Republic war effort. - SWTOR, SWTOR:E, SWTOR:A
5.Satele Shan
Canon: Grand Master of the Order, found Tython through the Force. Considered extremely powerful in the Force, impressed even the Sith with her sheer prowess. Defeated with ease entire squads of Force User killing droids, very powerful connection to the Cosmic and Living Force.-SWTOR, SWTOR:E, SWTOR:FA
6.Thon
Canon: One of the greatest Jedi Masters in the entire Old Republic era, masterful skill with Tutaminis variants, Sever Force, Battle Meditation and countless more techniques. A vanguard of the Order's principles and teachings. - TOTJ, TOTJ:C, TOTJ😖, POTJ.
7.Odan-Urr
Canon: A Jedi Master considered the leader of the entire Order, a thousand years of knowledge and training in the Force led to one of the most powerful Light Side connections on record by the Great Sith War, easily rag-dolled Exar Kun himself and his death along with other masters is considered one of the greatest defeats the Order ever suffered. - TOTJ, TOTJ:C, TOTJ😖
8.Arca Jeth
Canon: A venerable Jedi Master considered one of the greatest Battle Meditation practitioners of his time, a mastery of the Force gifted him the ability to contain Dark Side power and also eliminate masses of droids, considered a champion of his own people and the Order. - TOTJ, TOTJ:C, TOTJ😖
9.Jaric Kaedan
Canon: A master of the rare Juyo-Kos form, one of the finest swordsman of the Order in his time. Brought down the Dread Masters himself as a Jedi Knight with only a special ops team as support. - SWTOR, SWTOR:E
10.Vodo Siosk Baas
Canon: One of the greatest lightsaber instructors of all time and one of the finest Jedi Guardians on record. Also known for having incredible strength in the Force, a master of the highest of Force techniques. Four centuries of teaching culminated in the unfortunate rise of one of the greatest Dark Lords and Lightsaber masters of all time. Later appeared as a Force Ghost to vanquish the spirit of his former student. - TOTJ, TOTJ:C, TOTJ😖
Like I said this is not perfect by any means and I can imagine many disagreeing, I already know what question number one will be:
"Where is Revan?" given the on-going storyline in SWTOR, it is apparent that Revan in his prime has turned from the path of both Jedi and Sith, following the 'Reborn' path in place of either.
I have no doubt that both his Sith and Jedi incarnations would gain a place on either list, but as things are, he doesn't really fit on either.