Maha Vishnu

Acc. to Matsya Purana (Ch 47)

1. Narasimha dev in Satya Yuga
2. Vamana dev in treta yuga
3. King mandhatta in fifteenth treta yuga
4. King parsurama in 19th treta yuga
5. Lord Rama in 24th yuga
6. Krtavirya born in 25th yuga

Acc. to Vayu Purna from 36th chapter as mentioned above.

1. Rama avatara in 24th treta yuga
2. Narsimha avatara in fourth yuga
3. Vamana avatara in 7th Treta Yuga
4. Dattatreya avatara in 10th treta yuga
5. Mandhata avatara in 15th Treta yuga
6. Parsurama avatara in 19th Treta yuga

Acc. to Brahmanda Purana (2.3.73)

1. Dhruva in first treta yuga
2. Rama in 24 yuga
3. Varah in 4th yuga
4. Dattatreya in 10th yuga
5. Mandahatta in 15th
6. Parsurama in 19th

Acc. to Padma Purana

1. Rama in 24th yuga
2. Parsurma in 12th yuga
3. RAma in 27th (but not Dashrathi Rama)

Acc. to Devi Bhagavata Purana (4th book ch 16)

1. Dattatreya in 2nd yuga
2. Narasimha in 4th yuga
3. Parsurama in 19th yuga