The early returns from the 2015 rookie class thus far are giving a pretty good glimpse of what the NFL draft often looks like a few years out when you review a class. Sure, you'll get a number of the best overall players from Round 1, but the pie gets split, and plenty of big names come from Rounds 2-4 as well. Then you get the occasional star from day three. That's how this class looks so far. Half of the top 10 are first-rounders, but we've seen some breakout performers from Rounds 3 and 4 as well.

Here are the parameters, same as always:

This is a measure for all games this season, not just last week.

Total snap count matters. Staying on the field is a measure of value.

Positional value matters, but overall performance and impact on the team matter more.

Relative value matters. I ask: Would this player be a starter on most teams or on a good team?

Here's my latest ranking of the top 20 rookies in the NFL.

1. Amari Cooper, WR, Oakland Raiders

In terms of raw performance, Cooper probably isn't clearly ahead of Darby. I give him a slight edge because of his impact on the team. He's already arguably the most important offensive player on the Raiders aside from the QB. He's on pace for 88 catches and nearly 1,400 yards, and he hasn't dropped a pass in the past two games. He is, frankly, what we thought he was.

2. Ronald Darby, CB, Buffalo Bills

He's been the best player in the Bills' secondary and has officially reached doesn't-come-off-the-field status. He's doing a great job in coverage, and has active hands. He has nine passes broken up already, good for third in the NFL. Darby is the early leader for biggest steal in the draft.