ME1 and ME2 are both really fun if you're into sci-fi open world stuff. ME3 was so so, ending to the trilogy is complete crap. But I will say that ME3 has some absolutely brilliant things. One of my top three video game moments of all time was in ME3 (genophage cure mission for those of you who've played it).
You can tell that from ME1 to ME2 and then ME2 to ME3 it gets overly simplified and watered down and there is less and less work put into the game, something known as the EA Effect.