Oppenheimer's Kotowski may be even more bullish on Citigroup than O'Connor. He notes it is the cheapest stock on a price to tangible book value basis versus its historical average, and argues that discount is mainly attributed to its mortgage exposure. "In short, what we think is the single biggest reason for the largest discount to tangible book value in the group is the area that the market is telling us is improving most rapidly," Kotowski writes, referring to Citigroup. -- Written by Dan Freed in New York. Follow this writer on Twitter.