The minor league leaderboards can always be a bit dicey to look at. While it's a little too early to definitively view the current sample size as large enough, there are still interesting things to note which may be able to help you keep a step ahead of the other owners in your league. With that in mind, let's look at the leaders at each level in the minors in walk rate.
AA Leader: Rowdy Tellez, 1B, Blue Jays - 22.9% (Pre-Season Rank: #7 at 1B, #7 TOR)
Tellez was drafted by the Blue Jays in the 30th round back in 2013 despite being viewed as a top 20 high school prospect due to concerns about his signability. The Blue Jays were able to get him signed, and he's been slowly plodding through the minor leagues, reaching AA this season at the age of 21. While the 23% walk rate is clearly inflated, he has posted rates of at least 7% at each of his stop as a professional, and has kept his strikeouts at a reasonable rate to go with it. He possesses above-average power potential, but there are questions about how much of a batting average he will provide. If he can provide at least a .240-.250 average, he could be useable in a number of formats.
AAA Leader: Jordan Patterson, OF, Rockies - 21.3% (Pre-Season Rank: Not Ranked)
Patterson doesn't get the same love as some other Rockies' outfield prospects, but he is still an interesting player who would be more well known with other teams. The 21% walk rate is out of character (he had a 5% rate last season between High-A and AA), but the overall production has been solid at every stop. He hit .297/.364/.543 last year with 17 home runs and 18 stolen bases, and the advanced statistics point to a player who wasn't just taking advantage of high-offense environments. While the California league is known as an offensive paradise, wRC+ helps to view the stats as park-neutral, and Patterson posted a 153 wRC+ there, and followed it up with a 143 wRC+ in New Britain. He's a bit older for a prospect at 24 this season, but could be an interesting call up given that he can play first base and the corner outfield slots, and would do so for half his games in Coors Field.
High-A Leader: Mike Papi, OF/1B, Indians - 23.3% (Pre-Season Rank: Not Ranked)
Papi posted an excellent walk rate last year in his first go-around in High-A, but the overall numbers don't appear to show much else. The standards don't really jump off the page (.236/.362/.356, 4 HR in 127 games), but playing in the Carolina League will deflate offensive production fairly strongly. That line was still good for a 117 wRC+, and he's come out on fire this year. So far, he's matched his home run total from last year, and has a 1.063 OPS in 16 games. He's an interesting name in deeper formats given that he likely profiles as a first baseman defensively, and could be in line for a promotion fairly soon.
Low-A Leader: Steve Laurino, 1B - 21.8% (Pre-Season Rank: Not Ranked)
Laurino was a 25th round pick last year out of Marist, and is currently hitting .167/.364/.214 as a 23rd year old across 14 games at Low-A. Laurino is the reminder that the sample size is still small and not to just look at the leaderboards for ideas on prospects.