In all there are more than 10 forms of Hanuman. They are mainly classified as Maha-Veer Hanuman and Das Hanuman/Ramdut Hanuman. They include the forms of Hanuman, which, I have listed below:
1. Das Maruti – Hanuman with a white complexion meditating on Shri Rama and Sita Maa. This form is said to be the most peaceful form of Hanuman and any Sadhana dedicated to this form gets Siddh without any doubt. For Hanuman Chalisa, if one meditates on this form of Hanuman, there will be nothing in the world that will be impossible for the Sadhak. This form is mainly useful by the Grihasth. This form is only utilized in Nishkam Karma. Females can worship this form of Hanuman.
2. Sankatmochan Hanuman – This form of Hanuman has a mud red or vermilion complexion and is a left facing Hanuman with a Gada in the right hand and a mountain in the left hand. This form is the best for getting rid of problems and for Sakarm Karma and second best for Hanuman Chalisa. The person who meditates on this or the above mentioned form of Hanuman and recites it's Chalisa, will never face dangers in life. This is a fact. Females can worship this form. But sometimes certain Mantras or Sadhana dedicated to this form cannot be done by them.
3. Flying Hanuman – This is a very beneficial form of Hanuman. This form can also be mediated upon if someone faces problems in life.
4. Dakshin Mukhi Hanuman – A fearsome form of Das Hanuman. This form is south facing. It has a mud red complexion like that of the 11 Rudras. This form is best for destroying enemies and subduing dangers and getting victory in court cases, etc. This form is best for removing Abhichaar and other Tantrik activities. This mud red complexion form is a Tantrik form of Das Hanuman. While a white colored Dakshin Mukhi Hanuman is mainly for Rajasik Sadhana. Sadhana done by meditating on a South facing Hanuman is mainly meant for men. However, there may be some exceptions.
5. Panchmukhi Hanuman – The most beneficial form of Hanuman for Sakaam Karma. This form fulfills all kinds of wishes. It removes poison, gives success in life, removes ghosts, Pishach, gives mental strength and makes one fearless, gives wealth and health, attracts all the people in the world.
6. Saptamukhi Hanuman Kavach – An extremely Ugra form of Hanuman. The only Mantra/ Stotra that I could find was a Sapta Mukhi Hanuman Kavach. This Stotra can only be recited by those who have really really strong devotion for Hanuman or by those who have been Hanuman Upasaks for years. The Stotra is very very strictly not meant for anybody else. If this Stotra is recited by those who lack very much devotion towards Hanuman or haven't been long time Hanuman devotees, they will go to hell. This is mentioned in the Stotra itself. Other than the benefits mentioned in Panchmukhi Hanuman, this Stotra gives Mukti as well as health.
8. Ekadash Mukhi Hanuman – This Hanuman is a form of Hanuman, which has 11 heads. The person who worships this form will get rid of great sins like Braham Hatya and Gau Hatya.
9. Nav Khand Hanuman – The most powerful and the most fearsome form of Hanuman. Nav Khand Hanuman means Hanuman whose size is 9 times the size of 7 continents. This form has 1000 heads, 1000 hands with innumerable Astras in his hand.
10. Hanuman as a Veer – This form is of Hanuman as a one of the 51 veers. Veer Upasana is the second most fearsome Upasana after Sharabheshwar and Pratyangira Upasana. Invoking any Veer is equal to challenging your death. Because Veers are known to even cause death of their Upasaks. Hence, only the most deserving and fearless person can do his Upasana. This form of Hanuman is also called as Mahavir by many and among the Veer he is called as Rudra Veer or Kapilumbir Veer.
11. Hanuman as a Mahaveer – This form is of Hanuman as a Mahaveer. Here Mahaveer does not mean one of the 51 Veer but Mahaveer means a valorous and aggressive form. This form means the one who is valorous and destroys enemies, attracts the enemies and gives victory over the earth. His Upasana itself is only for men, Then, chanting this form's Mantras are a different thing.
12. Hanuman as a Rudra Avtaar – This is also a Mahaveer form. This form has a complexion like that of Rakt Chandan. This form is my dearest forms of Hanuman and there is nothing that remains impossible in all the three worlds for this form of Hanuman. This form is very specially beneficial for Hanuman Maha Mantra Sadhana.
The person who gets Deeksha in this form of Hanuman and for Hanuman Maha Mantra is without any doubt the luckiest and most favored Sadhak in this entire Shvet Varaha Kalpa. because Hanuman himself is the upcoming creator and the ultimate truth. This form of Hanuman is the ultimate and all the other forms come from this truth. This form and its Maha Sadhana is only for men.
13. Vichitra Veer – This form is the Veer form of the fearsome Panchmukhi Hanuman. Vichitra Veer Hanuman Mala Mantra is the only Mantra which I have found till today.
14. Ghatikaachal Hanuman(घटिकाचल हनुमान) - Not much is known about this form of Hanuman or about his looks and complexion. However, there are 2 Ghatikaachal Hanuman Stotras dedicated to him
15. Female Hanuman – This is special form of Hanuman where Hanuman is in a female form. There is a temple of it somewhere in middle India. This form of Hanuman is true.
16. Kasht Bhanjan Hanuman – A form of Hanuman whose temple is in Gujarat. This form is best for reducing Shani Peeda. The Kasht bBhanjan Hanuman and its Mantra which is very very popular in this temple are true.