Mass Effect is a really great RPG in the style of Knights Of The Old Republic set in a universe every bit as rich as star wars or star trek. As a scifi nerd it's kind of cool to be able to personalize your own flash gordon to save the galaxy with.
Mass Effect 2 is a really awesome shooter with a few RPG elements sprinkled in. It features a really great plot (that has almost nothing to do with the first game for some reason), a great cast of characters, and much tighter and more focused shooter gameplay.
Mass Effect 3 is a really great game when played in sequence from 1-3. Its gameplay is very similar to Mass Effect 2, and your choices in the previous games are actually given some weight in terms of gameplay. A lot of people dislike the ending, so there's that.
I think they're all amazing honestly. Nothing cooler than combining captain Kirk, Luke Skywalker and Captain America into 1 character who's job it becomes to save the galaxy from Reapers.