In many ways, the Reapers are the linchpin that holds the entire original Mass Effect trilogy together. They're a race of synthetic starships that have either been the main antagonist or directly involved with the main antagonists in all three games. The original creators of the Citadel and the Mass Relay, these ancient lifeforms allow the other sentient races to grow, thrive, and develop in order to harvest them in a repeating cycle of galactic purges. Therefore, the point of the entire initial trilogy is going to war with these machines to end this repetitive mass slaughter.
That being said, the games have no shortage of enemies and antagonists who are not part of the Reaper race. Some of them are just as formidable as the Reapers, while some of them are even more intimidating and challenging in a direct firefight.
10 Cerberus
You would think that a human-survivalist group would be an ally of Shepard’s and humans at large in the Milky Way. Unfortunately, Cerberus is a penchant for terrorist activities, murdering other alien races and experimenting on them, which makes them a villainous group.
What’s more, they’ve also engaged in seditious activity against their fellow humans, and have rallied against the Alliance. The group is well equipped militarily too, and is very organized, being led by a certain man who we will talk about a little later.
9 Thorian
A mass of tendrils that essentially cover an entire planet is bad enough, but a mass of tendrils that can also use mind control to have others do its bidding is a massive headache.
Originally residing on the planet Feros, the Thorian is an ancient plant creature with telepathic abilities that shoots spores into the air – spores which, if inhaled, puts those who breathe them in under its spell. With basically an entire planet to fight against, The Thorian makes the odds feel almost insurmountable.
8 Husks
What makes the husks such horrible adversaries to fight is not their skill or their toughness. In fact, husks are mindless creatures that run recklessly at you and are actually pretty easy to subdue.
It’s the fact that these husks were once your allies. Impaled on large spikes by the Geth, these dead humans are transformed over time into zombie-like creatures. Knowing that they were once your brothers and sisters in arms makes it harder to pull the trigger, but with them sprinting forward with no hesitation, that trigger must be pulled.
7 Matriarch Benezia
An ally of Saren and mother of Liara T’Soni, Matriarch Benezia is deeply entrenched in the first Mass Effect’s plot without even being prominently featured in the game up until you encounter her — and what a battle that is.
A powerful Asari biotic, she starts the fight by freezing Shepard and his crew in place while she assembles her team of Geth commandos. Probably the toughest boss in the entire game, she uses a range of biotic attacks and powerful defensive shields, as well as her Geth crew, to make life very difficult for the player.
6 Kett
Let's move away from the original trilogy for a moment to look at the Kett. A species found in the Andromeda galaxy, they possess the intelligence and physical characteristics of humans but with a tougher exterior, having the strength and bone density similar to the Krogans from the first three games.
Being extremely militaristic by nature and masters of biotics and medical technology, the Kett use a formidable combination of brute force, tactics, and curing capabilities to fight their enemies.
5 Archon
The main antagonist of Mass Effect: Andromeda and a high-ranking member of the Kett people, you know that anyone who presents the player with two simple options — surrender or burn — is a force to be reckoned with.
The leader of the Kett in the Heleus Cluster, the Archon has similar aspirations and characteristics as other antagonists in the other games. He believes that his alien race is superior to all others, and is willing to slaughter, subjugate and conquer other worlds in order to secure his and his people’s dominance.
4 Geth
Going all the way back to the first game again, the Geth are an ever-present force in the original trilogy. A race of artificial intelligence, they were originally created by the Quarians as machines of labor. However, after becoming sentient, they went to war with their creators, beating them to near-extinction.
There are many things that make the Geth so dreadful to fight. Among them are their cold demeanor, the various forms they come in, and the fact that they are often controlled and utilized by nefarious forces to try to destroy the player.
3 Krogans
While not always hostile to Shepard — teammate Wrex being a prime example of a reluctant friendly — the Krogans are one of the toughest and most intimidating races in the game, and that’s even when they’re on your side! Face a group of hostile Krogans though, and be ready for a tense, frantic battle.
With a rock-hard exterior often coated with armor, the Krogan can and will fire at you with weaponry. Even worse however are when they charge at you, as they are shockingly agile and can end you with a single strike.
2 The Illusive Man
As the leader of the Cerberus, what makes the Illusive Man so formidable is not his physical prowess, his intimidating aura, or his threats to burn Shepard alive. Rather, it's his mysteriousness and covert nature that makes him so dangerous.
Much like the Archon, the Illusive Man not only believes in human supremacy but wants to rule over all other races at any cost, even if that price is mass genocide or planetary destruction. The fact that you spend much of the second game working alongside the seemingly level-headed man makes him one of the series’ most sinister entities.
1 Saren Arterius
The original bad guy, Saren was the main antagonist in the first Mass Effect game and remains the most insidious and detestable antagonist in the entire series. Like the Illusive Man, Saren was once an uneasy ally of Shepard’s, as well as that of the Citadel Council, where he once sat as a Spectre.
Allying himself with Sovereign and the Reapers, Saren becomes totally indoctrinated, being used as a vessel to enact their galactic genocide. That’s why Saren is even worse than the Reapers; he uses his form to fool trusting aliens while aiding in their complete destruction.