One of my favorites was Mike Morse.
Saw him calmly turn around a 101 mph heater in on his hands in a game way back when he was first coming up...I think it was 2006...and confidently predicted he'd be a star hitter in the big leagues if he could field enough to stay in the line-up. Took him a few years to prove me right. :)
Actually, my all-time best pure scouting call for hitting, though, wasn't even for a Mariner. I have a group of friends who grew in the NYC area, most of whom are Mets fans. There was a period of time where I had a 16-game plan with them at Shea Stadium/Citi Field and we'd take in some weekend games in person. Back in the spring of 2009, we went to an exhibition game at Shea right before the start of the season and this young player they had who had no position and no guaranteed line-up spot but who'd had a nice little cuppa in 2008 was the topic of conversation. I took a look at his swing and said: "that guy is going to hit .330+ and develop 20 HR power when he gets it figured out." They laughed at me. That player's name was Daniel Murphy. BTW, when the Mets let Murphy get away in free agency and chose to trade for Neil Walker instead, I told a coworker/Mets fan at the office that he would not be so happy about that parlay in short order, that Walker was a BIG downgrade. He thought the opposite. Murphy is a minus fielder and, at that time, was a solid, but unspectacular hitter by the numbers. My clue was the sudden HUGE drop in K rate. The rest is history. :D
On that same 2009 team, there was this little waterbug outfielder who I said should be leading off instead of Reyes. Reyes got hurt and he went right to the top of the order and was a nifty little sparkplug. That was Angel Pagan, who went on to have some nearly AS level seasons with the Giants and Mets.
In terms of the Mariners, the worst call I've made was confidently predicting that Olerud would bounce part of the way back after 2003 and that Boone would bounce back after 2004. That pair of blunders still smarts.
But I'm sure I'm forgetting other moments of stupidity. Most recently, I thought Zunino was going to break out. Oops.