The various new 737's are tough ... don't ask me

The various iterations of the DC-9 are easy once you get the hang of it.

DC-9 - it's small
MD-80 - looks like a DC-9, but is noticeably larger
MD-90 - looks like an MD-80, but its engines are noticeably larger
717 - looks like a DC-9 (noticeably smaller), and it's engines are noticeably larger.