Morgan Stanley believes now is an opportune time to buy shares of Frontier . "With the SAVE merger pursuit now in the rear-view mirror, we view Frontier as the quintessential ULCC (ultra low cost carrier) due to its ultra-low fares, ultra-low cost structure, and attractive (normalized) margins," wrote analyst Ravi Shanker in a note to clients Wednesday. Shanker resumed coverage of the budget airline with an overweight rating, noting that low-cost carriers across the board should continue to benefit from pent-up demand for travel and easing fuel costs. At Frontier in particular, Shanker believes the shares are trading at attractive levels as he expects the company to generate strong nonticket revenues and notch a fresh ancillary revenue per passenger record boosted in part by capacity growth and higher available seat miles. According to Shanker, "ULCC valuations more than reflect the near-term capacity constraints, while giving the space little credit for an extremely strong topline and (relatively) easing jet fuel inflation pressure, setting up very attractive risk-reward." Shares of Frontier have slid more than 6% this year and 12% since the start of August but could rally 57% from Tuesday's close based on the bank's $20 price target. — CNBC's Michael Bloom contributed reporting.