If it's all about length, then put the Toy Cannon on the list. He'd belong there just as much as Allen.
But if the question is "Is there a player who, in his prime, would cause you to drop everything just to watch him take an at bat?" - Gotta say that my preferences will lean towards the guys who were not only sluggers, but great all around hitters.
Pujols, Howard, Mantle, Mays, Griffey, Aaron: these guys were great all around hitters.
Reggie, Strawberry, McGuire, Allen: these guys were all one dimensional sluggers for the most part.
Bonds? Lord knows what he would have been without the chemical assistance.
Evan White - 1B drafted out of Kentucky. Currently hitting .277/.345/.532/.877 for the AquaSox
Gone But Not Forgotten (former adopt-a-players):
Forrest Snow - Signed a minor league deal with the Brewers. Split time between AA and AAA this season.
Alex Jackson - Traded to Atlanta, moved back to catcher and finally hit well in High-A ball. Now hitting .091/.412/.182/.594 in AA
Eric Thames - Slowed down after a hot start. Hitting .253/.373/.537/.910 for the Brewers.
Matt Mangini - Out of baseball. Now sells cigars.
Mike Morse - Back with the SF Giants but hasn't played since May due to injuries. Looks like time has caught up with him.
Jamal Strong - Let go after 2005. Played in the Yankees, Cubs and Braves systems. Now a national scout for the Cards.