Harvard research for a rainy day in the off season. This suggests that OL is the safest way to go--- (not new--2 year old study)
Why this source says that WRs are notoriously bad first round choices (2006):
Just some interesting reading. If you are a research scholar, you may find interest in this study, but if you are not a consumer of quantitative data analysis, this one might not interest you. It basically explains that there are too many variables at play for the experts to succeed in drafting WRs, but it has a few interesting points, such as which measures are the best at predicting a WRs success (college performance over combine and pro day measures).
Finally, I ran across this--
The "new and improved" draft value for each pick (2011):