Good points, MikeH92467. More on the subject of...
A good mic is important
, but it's not the only factor. Not just the mic is important. What you're recording to and the sound quality of the space you're in is also important.
I've in other threads mentioned using heavy blankets to deaden echos of set flats and walls not in shot. When we shot Stagecoach In the Sky we have to put a blanket over a flag to cover an open hatch on the plane because the echoing was nuts if we didn't.
Number of Mics
One mic often isn't enough. Sometimes you need two mics to get all the sound in a shot, particularly if you have a scene where actors are far apart in the frame and there's overlapping dialogue.
What You're Recording To
Furthermore, a lot of cameras, and particularly DSLRs, have crappy audio, so on such cameras you definitely want to be recording second sound to another device of higher quality.
The Slate Clap
A lot of people don't realize how important this is. That clap sound helps you sync or resync audio to picture if you don't have a slate that generates timecode. It's particularly important if you're shooting separate sound, as you don't want to be manually trying to sync sound to picture in post...trust me.
One trick a DP I worked with taught me is to always use a stereo mic and to set the levels so that the left channel is "ideal" and the right is maybe 10-15% lower. The right channel ends up being the safety track if the sound on the left channel spikes and distorts or pops. On more than one occasion it's saved me from having to go back later and loop lines.