Having done some research Milwaukee actually has decent attendance numbers (avg 33,632 80.3%). The team with the worst attendance in baseball is the Florida Marlins (16,959 44.0%) but the AL already has a Florida team that pulls in shitty attendance numbers in Tampa Bay at #29 (17,954 52.7%). That's for a contending team! MLB would be wise to get those teams to relocate as Florida is a shitty sports state.
Houston, the team they want to move to the AL, is a middle of the pack team (#16 25,367 61.9%). The NL teams between Houston and Florida are San Diego (#17 24,306 56.9%), Cincinnati (#18 24,008 56.7%), Arizona (#19 22,910 47.1%), Washington (#21 21,402 51.6%) and Pittsburgh (#23 20,747 54.1%). You can't move the Reds or Pirates because of they are ancient teams with a long standing NL tradition. San Diego is intriguing and they'd still have two California teams to play in the AL. Angelos in Baltimore would probably shit bricks if the Nats were moved to the AL so avoid that mess. Arizona is looking like a good choice.
So I say either Milwaukee, San Diego or Arizona.