Mass Effect needs good boss battles.

I did enjoy fighting architects, specially the one on Voeld where you need to watch out for your life support. But I gotta admit it was kinda repetitive, but it had its charm (unlike many OT boss fights. IMO)

In the original trilogy we had boss fights like:

1) Saren : Geth Hopper, anyone?
2)Shadow Broker : No challenge to me. Not a boss battle, but a boss that is on the battle, if you get what I'm saying
3)Tela Vasir: Nothing unususal here. Just keep hitting her while she biotic charges through the field. And you can even bypass the majority of the fight if you correctly time Liara's Stasis on her.
4)Tresher Maw : Again, no challenge. Cain? Yeah! No cain? Shoot him and not use the cover that can break. Even the Klixens were most challenging than it.
5)Shepard's clone, Citadel DLC: Now that's what I call a challenge. An enemy that regenarates shield and fight you with your own powers and tactics. Plus Rasa that uses Cloak.
6)Mirror Match : The best one ever. Pain in the ass, you gotta keep moving on the scenario, which makes you die a few times before actually knowing what you're up against where you are.

And yeah, I'm talking Insanity here.

And I did wish we had a final boss fight with the Archon himself, but the Relics were a pain in the ass, but there was no variety there. The same Nullifiers and the Architect head while defending point A, B, C or D.