I will say watch them in order they have aired, since the producer have made it so that you will get the most fun. With the exception of Kizumonogatari, which I think should have aired before Monogatari Season 2. If you want you can read it. The Light Novel is over at Baka-Tsuki.
People may argue that it's better to watch them in the order they aired, but I'm glad I saw Fate/Zero began to read Fate/stay Night.
That was because Fate/Zero takes place before Fate/Stay Night.
Here the actual order of Monogatari Universe:
Kizumonogatari
Nekomonogatari
Bakemonogatari
Nisenmonogatari.
Monogatari Season 2
The aired order is:
Bakemonogatari
Nisenmonogatari
Nekomonogatari
Monogatari Season 2
Kisumonogatari (?? when??)
You can watch it anyway you like, but watch Monogatari Season 2 ONLY after watching everything that has been released so far (katanagatari is exception since it's not related in any case, just the same author).
You will need the background of previous releases to understand monogatrai season 2 completely. So best thing is to go on the air of date release to avoid confusion.
