Easiest Kichaka
Bhima barely sustained any injuries from that bare hands fight and he had caught Kichaka off guard.
Then Dushasana
Bhima got injured by Dushasana's arrows but later killed him with his mace.
Next Jarasandha
Bhima wrestled with Jarasandh for 14 days and it was his longest fight ever. Jarasandh was strong and injured Bhima much.
Finally Duryodhana
Duryodhana knocked Bhima out unconscious 3 times in the final gada yudh and this was Bhima's toughest fight where he almost got killed. Later he crushed Duryodhana’s thighs.