Thorian was a terribly predictable fight. Shoot the node one, kill the husk that wakes up, shoot again, kill again, shoot again, kill again, explode the node, damp the Asari, get killed by the hundred husks behind you that you couldn't shoot because they were taking a nap.
ME1's combat was a mess. Biotics trivialized encounters, Immunity lived up to its name entirely, snipers would and could oneshot you the instant you saw them, both your allies and your enemies' AI was borked... I can't believe you'd honestly tout ME1's combat over ME3's power combo, weapon customization, and improved cover system.
ME2 and 3 were just so simple by comparison to 1, it's like, every room had ample cover, so you were pretty much playing Time Crisis but you had to move yourself. I mean, don't get me wrong, I really liked Time Crisis (1, 2, and 3). 3 I honestly felt like I was playing something else entirely, like some kind of cross between a bullet heck r type and an FPS in some sort of Frankenstein experiment. Again, I like Frankenstein, I like r types, but... meh.
Plus biotics didn't trivialize encounters until you got Liara substantially buffed (or you did it yourself), which was at least partway, immunity if powerful but not godly, snipers could not and would not oneshot you the instant you saw them, and I didn't really have many problems with the AI myself, not in the grand scheme of things or compared to other games.
ME3 was considerably less challenging in a real sense than 1, as was 2. Even if you relegate 1's difficultly primarily to the earlier parts such as that insane bar fight towards the beginning or trying to save Tali in the tunnel, it's still much more than 2 or 3.
I replayed 1 at least 3-4 times.. 2 maybe once.. 3... once was enough. They're all good experiences but 1 offered the most game wise.