Mike Moore: 1981: Round 1, Pick 1, 17.8 (27.9) WAR
This was just a fantastic pick by the Mariners back in 1981. The only other real option at the top was Joe Carter, who was known for his prolific bat and walk-off heroics in the 1993 World Series for the Toronto Blue Jays.
However, Mike Moore actually had a much better career than Carter. He also had a better career than Ron Darling, who was taken 9th that year. Not just that, but he actually stuck around in Seattle for a while, and holds the current spot of third best first round pick to amass stats for the Mariners.
To go along with that, Moore actually has one of the best statistical pitching seasons in Mariners history by someone not named Big unit or Felix. His 6.3 WAR in 1985 is the 8th-best mark in team history. He followed it up with another good season in 1986 at 4.6 WAR. Check out his combined stats from those two seasons.
1985-86: 3.89 ERA, 71 GS, 513 IP, 25 CG, 1.312 WHIP
Sure, it might not seem that impressive, but all the innings that he ate up and the limited runs he gave up while doing so made for a really strong run from Moore. Especially on a team that just wasn't good, going 141-183 over those two seasons.
The team would'nt re-sign him after the 1988 season, and he would head south to Oakland. It was the start of a nice three year stretch where he started 101 games with a 3.35 ERA and 112 ERA+, while throwing 651 IP. He ended up being a real nice pick by the Mariners, even if he wasn't even the best pitcher that they took that year in the draft.