ME3 built on ME2's gameplay and improved it, making powers more versatile and generally making a fun combat experience into a great one, especially with the variety introduced in enemies and maps and the ekimination of heavy weapons except as weapons of opportunity. It is unfortunate that it also had the most annoying story components.
ME1's combat system is awful. The story and depth is great, but the gameplay in combat is repetitive, boring, and frustrating beyond measure in some places. And it had the usual RPG problem that Shepard was weak in the first part, making it difficult, and godlike as the game wore on, making it easy and boring.
Rebuild ME's story with ME3's gameplay and I am so there.