The most professional thing you can play is the melody. Now, don't get me wrong. I love, and I mean love experimenting with a song, turning it inside out, putting in blues licks, jazz progressions, and everything else but the kitchen sink, I hope that there will always be a time and place for that. But the longer I play (I'm almost 40--maybe that has something to do with it), the more I'm drawn to playing the melody of the song.
It's the thing that really makes that song unique. It's the part everybody can recognize instantly. I feel that if you establish the melody early in the song, you can start to vary it, and the audience can hang with you. If they can't tell what the song is, you've already lost them.
So what do I mean the melody is the "most professional" thing to play? Aren't professionals the best players, and the ones who can do the most? Yes, but often they choose to play a melody in a cool way. One of my favorite guitar kickoffs is Tim Austin playing the Lonesome River Band's "Listen to the Word of God." It's crosspicked, syncopated, and cool, and based on the melody of the song. Check it out!