Every express bus from the outer boroughs goes to Manhattan, but other than the X25 and X90 buses that were killed in 2010, why doesn't Manhattan have its own express bus nowadays? Here's my proposal for an express bus within Manhattan:
X70: Express weekday only service between Columbus Circle and Battery Park City
Southbound: via Central Park West, 63 St, 9 Av, 23 St, West St, Chambers St, and North End Av
Northbound: via North End Av, Chambers St, West St, 10 Av, 42 St, and 8 Av
Bus will run from 7AM to 8PM every 30 minutes. No stops on West St in both directions.