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.
Alex Jackson - Not very impressive for his second year in A ball. My boy is starting to look like a 1st round bust.
2016 Clinton: .243/.332/.408/.740
2015 Clinton/Everett: .207/.318/.365/.683
2014 Peoria: .280/.344/.476/.820
Forrest Snow - Started the year injured, then split the year between AA and AAA. At 27, his career may be winding down.
2016 AA/AAA: 3-4, 2.83 ERA, 61 K, 15 BB, 1.074 WHIP, 54 IP
Career: 33-47, 4.08 ERA, 628 K, 241 BB, 1.257 WHIP, 682 IP
Gone But Not Forgotten (former adopt-a-players):
Eric Thames - Battled injury and had a down year in Korea, only hitting .317/.425/.676/1.101 over 121 games.
Matt Mangini - Out of baseball. Now sells cigars and offers baseball training/evaluation services.
Mike Morse - Released by the Pirates in April. Looks like the end of a 12-year career for the former M's prospect.
Jamal Strong - Let go after 2005. Played in the Yankees, Cubs and Braves systems. Now a national scout for the Cards.