Edilio Colina has probably been the best player so far. His EqA of .276 is the best on the roster, helped by a team high .289 BA and .358 OBP. Colina also leads the team in runs (39) and RBI (30), and has a relatively low strikeout rate (38 in 245 at bats). For a 19-year-old second baseman, these are actually pretty good numbers. He doesn't have any power, but neither does anyone else on this team.
The guy with the most upside is probably Denny Almonte, the Mariners' second-round draft pick in 2007. Almonte had a rough go of it in 2007, hitting .145 in 76 at bats between Arizona and Everett. But he's starting to pick it up as the season progresses this year. He put up .275/.339/.461 in June --- not earth shattering, but an indication that there's something to get excited about. He strikes out once every 2.7 plate appearances and doesn't walk a whole lot to offset that. If Almonte continues to improve, he should be on pace to advance to high A next year.
I don't know if I can say that about any of the other Rattlers. Ronald Garth is in his third season at this level and hasn't shown marked improvement. He hit 18 homers in 594 at bats in 2006 and 2007 with Wisconsin but has only three this year. Gavin Dickey is in his second season in Wisconsin and has improved from a .668 OPS last year to a team-high .748 this year (though he's only played 46 games). The right-handed Dickey actually hits righties much better than lefties, posting a .288/.349/.417 line against them. Dickey has good speed, but is just 10-for-19 on stolen base attempts. Alex Liddi showed promise as a 17-year-old in the Arizona league, but through 640+ at bats in Wisconsin over two seasons he hasn't had much success. His .251 EqA puts him below average even in the light-hitting Midwest League. Liddi did have a nice June, though, with a .312/.365/.403 line. Maximo Mendez is an intriguing player. He strikes out a lot (78 in 255 plate appearances) but leads the team with 28 walks. He's speedy (12 SB in 16 attempts) but is listed at a stringbean-like 6'2", 150 lbs. Juan Diaz is a nice shortstop defensively, but has dreadful offensive numbers (.244/.285/.297) overall. Joe Dunigan (.183/.253/.307) can't have the season end soon enough.
Overall, it's hard to really glean many positives from the numbers posted by the Timber Rattlers this season. Fox Cities Stadium has not historically shown itself to be a complete dead-ball stadium relative to the rest of the league, so it's hard to dismiss the dreary offensive stats as a product of the ballpark. There was a lot of cold weather in April, to be sure, but nine Wisconsin home games were either postponed or outright canceled due to weather. One possible explanation can be the youth of the team. Only Lansing has given more at-bats to 19-year-olds than the Rattlers. Burlington, Dayton, Kane County, and South Bend have used no teenage hitters this year, while the Rattlers have four regular starters who are 19. The Mariners are known for quick promotion, and it is really apparent at this level. The wisdom of this approach can be debated --- do you want guys thrown to the wolves, in over their head but challenged to improve quickly (baseball Darwinism, so to speak)? Or do you want your hitters to dominate at a lower level to build confidence and consistency?