No doubt, Mass Effect blows ME2 out of the galaxy when it comes to sidequests. Not only is it simply a matter of quantity (there are about twice as many sidequests in the first game), but for the most part they're a bit...mmm, beefier, I guess. Don't get me wrong, Mass Effect 2 has some great ones- the first time I wandered around the wreckage of the Estevanico, worried it was going to teeter off the cliff, I frequently held my breath. And checking out the Normandy crash site? Pretty haunting. But too often the ME2 sidequests were just go here, set a timer, run! , and as a result they felt a bit like padding...not that I don't do them all during every run.
The events in Mass Effect were far more engaging, I thought. Wiping out pockets of geth infiltrators, helping to defend an outpost from rachni invaders, negotiating with a cult leader, investigate the nefarious dealings of Cerberus...all more substantial than simply, I don't know, turning on a shield generator. But more importantly, nearly every ME sidequest had dialogue, which infinitely helps to make them engaging. In the sequel, Shepard and her pals were nearly silent through every sidequest. It's jarring. And it stinks!
My favorite from either game? I Remember Me, which is available to a Shepard with a colonist background. Shepard is charged with talking down a recently-rescued slave from committing suicide. It's heavy stuff, very well-written and completely heartbreaking.