[UPDATE 2]: Added Owarimonogatari.
[UPDATE]: Added Hanamonogatari and Tsukimonogatari.
With already four completed seasons, another to be aired later this year and a movie that will premiere sometime in the future, it can get a little confusing what is the correct order to watch all Monogatari episodes. These are the most common proposals:
By airing date:
1. Bakemonogatari (15 episodes)
2. Nisemonogatari (11 episodes)
3. Nekomonogatari: Kuro (4 episodes)
4. Monogatari Series Second Season (26 episodes)
5. Hanamonogatari (5 episodes)
6. Tsukimonogatari (4 episodes)
8. Kizumonogatari (Premiere date not confirmed)
By chronological order:
2. Nekomonogatari: Kuro
5. Owarimonogatari (Ougi Formula, Sodachi Riddle, Sodachi Lost)
6. Monogatari Series Second Season
And lastly, I will propose a third option. This one follows the publishing order of the light novels by Nisio Ishin.
By light novel publishing date:
4. Nekomonogatari: Kuro
5. Monogatari Series Second Season (Nekomonogatari (Shiro), Kabukimonogatari)
7. Monogatari Series Second Season (Otorimonogatari, Onimonogatari, Koimonogatari)