While your fantasy league won't be won or lost solely on what happens during the draft, a good draft is a great head start for the rest of the season. If your fantasy league is a competitive, exciting one, the other owners in the league probably follow baseball as closely as you do, meaning that your biggest edge will be your preparation.
While cheat sheets are always an excellent tool, you need to know more than which player deserves to be drafted when; you need to know what your friends/enemies are doing. Sadly, most owners won't tell you exactly where they plan on drafting players, so knowing each player's average draft position (ADP) serves as a useful proxy for that information. You may think a newly minted closer is an undervalued player, but if the community of fantasy players agrees, you may not get much a chance to use that knowledge.
One tool I like to use for myself is the yearly ZiPS projections. Adding in playing time -- knowing how players will be utilized is a weakness of computer projections, even complex ones -- I make my own ZiPS fantasy rankings and then compare them to ADP. Like any tool, you shouldn't stick to a rigid plan and follow it to a T, but having as much information at your disposal as you can is what wins leagues.
Below, I've listed some of the hitters for each position who are either overvalued or undervalued when comparing ADP to my ZiPS fantasy rankings, noting the players that I find to be the most interesting. We'll get the pitchers in the next file.
Catcher
Overvalued: A.J. Pierzynski, Boston Red Sox
ZiPS Rank: 448 ADP: 226
You've watched the other owners overdraft the top tier of catchers, underestimating the higher injury rates of players donning the tools of ignorance. Now you're trying to get a catcher on the back end of the draft and your eyes land on Pierzynski, the 12th catcher taken on average in ESPN drafts. Keep looking. Pierzynski remains a 37-year-old catcher who gets a lot of value from his homers. And he's moving from playing in excellent home run parks with the White Sox and Rangers to a park that's death to lefty home run hitters.
Undervalued: Wilin Rosario, Colorado Rockies
ZiPS Rank: 41 ADP: 111
ZiPS sees Rosario as an average catcher overall, but he happens to be an average catcher playing half of his season at Coors Field. ZiPS suggests that he be the No. 4 catcher off your board, and while I wouldn't take him No. 41 overall as ZiPS suggests, his projected values put him slightly above Brian McCann or Jonathan Lucroy, both being taken a half-round to a whole round earlier. If I'm thinking catcher with a pick in the late 90s or if McCann and Lucroy are off the board, I'm strongly looking at Rosario.
First Base
Overvalued: Prince Fielder, Texas Rangers
ZiPS Rank: 73 ADP: 13