Sell-side analysts have swung behind National Australia Bank following last week's interim profit reports from NAB, ANZ and Westpac. Several analysts have upgraded NAB to a buy and others have made it their preferred big bank stock.NAB's return to favour has been matched by Westpac's fall from grace. Few analysts have it as a buy and at least one, Deutsche Bank, downgraded it last week.The UBS report on NAB's interim result noted that it was the first in several years that was not "clouded by mishaps or legacy issues".NAB reported pre-provision earnings growth of eight per cent, compared with average pre-provision growth of 4.2 per cent among its peers.The term most often applied to NAB was that it has "momentum". Goldman Sachs estimated that it would be able to maintain above-system loan growth for another couple of years.Citi cited "good momentum in the franchise" in upgrading NAB to a buy. It said: "The bank has the best operating trends - flat costs, solid revenue growth."A couple of analysts pointed out that NAB had the most to gain from a recovery in business lending, given its strength in that market and the big contribution business banking makes to its bottom line.It was not all good news for NAB, however. UBS upgraded its recommendation to buy but cautioned that it was worried the bank was growing its mortgage book at 2.5 times system just as arrears are rising.It was also worried that NAB's productivity program, NextGen, did not appear to be bearing fruit.Merrill Lynch said NAB was yet to show that its aggressive retail strategy was sustainable.While NAB was seen as having momentum, the feeling about Westpac was that it was "constrained" and "sluggish". Goldman Sachs said sluggish growth in New Zealand, institutional banking and Australian retail would hold it back beyond the current financial year. Citi described Westpac's result as a "pretty uninspiring revenue performance". It also expressed concern that the bank's level of deposit-funding, at 52 per cent, was too low.On the positive side, several analysts said Westpac's efficiency (it has the lowest cost-to-income ratio among its peers) would be a real advantage in a low growth environment. UBS said Westpac's cost management was a "stand-out".ANZ attracted positive comment for the acceleration of growth in Asia. Goldman Sachs said ANZ would be able to tap two sources of growth to compensate for the weak Australian credit market: global markets' operations in Asia, and wealth management, following the buy-out of ING from the ING Australia joint venture.