Let me get my smart-ass instinct out of the way first: journals have impact factors, not papers :)

But it doesn't really matter because it's a somewhat-empty vanity-metric anyway. For assessing clinical trials (and aggregating their results) the Cochrane collaboration is probably the gold standard. Here's something I found on their method to evaluate trial quality: http://www.bmj.com/content/343/bmj.d5928