2008 data; only pitches with an n of greater than or equal to 100 are included. The numbers are based off of Josh Kalk's PITCHf/x tool, which is both a blessing and a limitation.
Top 5 individual offspeed pitches by swinging strike rate:
Changeup, Mark Lowe: 29.9%
Changeup, RRS: 19.8%
Slider, Felix: 18.8%
Slider, Tyler Walker: 18.4%
Changeup, Tyler Walker: 18.1%
Bottom 5 individual offspeed pitches by swinging strike rate:
Changeup, Jarrod Washburn: 4.5%
Curveball, Felix: 4.6%
Curveball, Roy Corcoran: 6.7%
Slider, Carlos Silva: 7.5%
Changeup, Carlos Silva: 8.0%
Top 5 individual offspeed pitches by strike rate:
Changeup, Carlos Silva: 69.1%
Changeup, Mark Lowe: 68.4%
Slider, Tyler Walker: 66.6%
Curveball, Erik Bedard: 66.1%
Changeup, RRS: 63.4%
Bottom 5 individual offspeed pitches by strike rate:
Changeup, Jarrod Washburn: 46.5%
Slider, Carlos Silva: 46.6%
Changeup, Miguel Batista: 50.3%
Curveball, Roy Corcoran: 52.5%
Slider, David Aardsma: 52.9%
Top 5 individual fastballs by swinging strike rate:
Brandon Morrow: 15.5%
David Aardsma: 9.2%
Erik Bedard: 7.8%
RRS: 6.8%
Roy Corcoran: 6.6%
Bottom 5 individual fastballs by swinging strike rate:
Garrett Olson: 2.1%
Miguel Batista: 2.8% (cutter)
Carlos Silva: 3.2%
Jarrod Washburn: 3.6% (cutter)
Tyler Walker: 4.5%
Top 5 individual fastballs by strike rate:
Brandon Morrow: 67.5%
Jarrod Washburn: 66.0%
Carlos Silva: 66.0%
Felix: 65.9%
Tyler Walker: 63.5%
Bottom 5 individual fastballs by strike rate:
Mark Lowe: 58.5%
Miguel Batista: 58.7% (cutter)
Miguel Batista: 60.9%
Roy Corcoran: 61.3%
Jarrod Washburn: 61.4% (cutter)
This whole data dump came out of my wondering which is the best individual pitch on the roster, and looking at the numbers, I think the big contenders are Mark Lowe's changeup, Brandon Morrow's fastball, and Tyler Walker's slider. Unfortunately for the latter two, as much as I love a good competition, there's no way for me to even begin to pretend that this is a difficult choice. If this were a footrace, Walker's slider and Morrow's fastball would be in snowshoes while Lowe's changeup would be wearing rollerblades with afterburners on them. That pitch just blows all other ones away. The most missed bats and the second-best strike rate? It's as dangerous and precise as a sniper rifle. Lowe's changeup got more swinging strikes than Cole Hamels' and Johan Santana's. Over half of the swings batters took at his changeup missed the ball.
...too bad it isn't paired with anything else particularly special. Lowe has a sharp slider, but it isn't very tricky or accurate, and he hasn't gotten nearly as much out of his fastball as you'd expect from someone who can regularly touch 95-97. His fastball command last year was even worse than David Aardsma's. It's weird to see a guy do so much with his change while struggling with his fastball, given the similar arm actions, but in this case, there's no denying what's real, and if Lowe intends to step up and become a trusted high-leverage reliever, he's going to need to settle himself down and start locating his heater. If he does that, he'll be one of the most successful pitchers on the staff. If he doesn't, he won't end up in enough pitcher's counts to put his best weapon to proper use.
Mark Lowe's changeup: the best pitch on the Mariners. Over time I wouldn't be surprised to see Brandon Morrow's curveball start making its bid, since when it's working that pitch is about as awesome as clementines and lace lingerie, but the last I saw of it, it was still too inconsistent in break and command to stand as a real candidate. It's all upside, though, and if Morrow gets it figured out, then he'll be able to take that and his mind-blowing fastball a long, long way.
Garrett Olson's fastball is really bad.