Original: Mass Effect
The original Mass Effect was a great start to the trilogy. But even if you didn't agree with how Mass Effect 3 ultimately ended, you have to admit that Mass Effect 2 improved on the first game in more ways than one.
Driving the Mako around in the first game could be fun, but ultimately it got tedious. Luckily it was left behind in Mass Effect 2, which overall upped the pace, not to mention the stakes. Exploring character-specific storylines while preparing for the ultimate suicide mission was a great structure.
And the combat was loads better in Mass Effect 2, with abilities and powers playing nicely with weapons, and ammo no longer being a finite resource. That gave you the freedom to loosen up and have a little fun during firefights, something that rarely happened in the original.