In order from worst to not-all-bad:
Strongly-ideological, highly-committed gun prohibitionist: Harold Koh.
Extensive record of anti-Second Amendment leadership: Secretary of State Clinton, Sen. Sheldon Whitehouse, Gov. Deval Patrick.
Mixed record, but with very little positive: Amy Klobuchar.
Mixed record: Cass Sunstein, Janet Napolitano [negatives include the ridiculous Dept. of Homeland Security report conflating political dissent with terrorism; as governor she signed some pro-right to arms legislation, and vetoed other bills], Jennifer Granholm [like Napolitano, a mixed record as governor, including signing some important reforms].