At the risk of putting meaning where there isn'y any and giving out backhanded complements, I nominate Gio Gonzalez as the best 5th starter in baseball. He is 6-3 with a 3.58 ERA and I don't believe he has missed a start. His stuff is awesome and last outing on sunday he threw the most pitches in a game in his career.
I didn't go through the other 29 teams to see if there is another 5th starter who is better, but I think I have a feel of what you usually get with a 5th starter. A back of the rotation guy is a guy who is sort of an innings eater who will give up a start if there is an off day to keep the other starters on every 5th day. Often there will be multiple guys who will get that 5th start in a season as the team tries to find a pitcher to fill that spot. When the big 3 were pitching in Oakland it was sometimes hard to figure out who the number 1 guy was, but you always knew who #4 and 5 were, one year #4 was Lidle, one year it was Lilly one year was Harden. Interestingly the years the A's were dominating after the all star break it was often because that #4 guy caught fire.
It's easier to talk about what a #1 starter is. A #1 starter is a guy who probably started opening day, always goes every 5th day, probably would start the first game of a playoff series or an important series, and would go up against other teams #1 starters. Lincecum is the Giants #1 starter. Sabbath Bloody Sabbathia is #1 for the Evil Empire, Beckett is #1 for the Red Sox, Johan Santana is the Mets #1 guy.
The A's #1 is Ben Sheets. He pitched opening day. He is a workhorse so far this year and going out there and taking the ball every 5th day, but not really showing elite status. But it's tough to be a #1. Gio is holding his own in the 5 spot, notice Gio pitched Sunday, and Sheets went monday, they are still cycling through the rotation 5 then 1. To begin the year the plan was the A's rotation was supposed to go Sheets, Duchscherer, Braden, Anderson, Gonzalez. But the best laid plans of mice and men sometimes dont' work out, the 2 and 4 spots have had trouble staying on rotation. But the 1-3-5 keep coming up and cycling through. And I think we oughta take a second to notice how good that 5 guy is doing.