First time kept before the first round: Buster Posey, Giants.
First time claimed at any time during the draft phase: Shawn Green, Blue Jays; Pat Dobson, Orioles
Orthodox (selection of the franchise's Top 5 rated over the years): Astros, Braves, Expos, Marlins, Padres, Phillies, Red Sox, Tigers
Marlins have the biggest dropoff (by my measure) between their #5 and #6 rated players (most obvious choice of first 5), followed by Padres and Expos tied.
Marlins have been used 10 times for which I have records, and 47 of their 50 first-round keepers have been Brown, Sheffield, Ramirez, Cabrera, Piazza. Piazza's been passed over twice and Cabrera once.