I think each friendship is unique in its own way..
Since the most focused friendships were those between Nidhi-Anji, Nidhi-Rohan and Ashutosh- Mallika, Ashutosh- Rohan... I am only going to consider those..
Nihdi and Anji/ Ashutosh- Armaan - a friendship that most of us have seen, heard of or experienced...a childhood friendship that continues into adulthood...where the mutual care, concern and affection are at their peak..so, there were so many beautiful shared moments between them...but nothing exceptional or extraordinary...
Ashutosh- Mallika - A longstanding friendship that stood the test of rejection...Mallika continued to remain friends with Ashutosh even after he rejected her love...but I sense a tinge of selfishness on both sides in this friendship...Ashutosh clung on to Mallika even after rejecting her love because she was the sole constant in his life...even if she could not fill the vacuum left behind by Baba, she still stuck around and offered solace when he needed it...so, without a thought as to what impact their continuing relationship might have on her, Ashutosh clung on to her...
Mallika was equally selfish...she knew her position is Ashutosh's life was unchallenged...even if he had rejected her love, he still remained her faithful companion...so she played the waiting game...she thought she could wear down his resistance some day or the other..so long as there was hope, she continued to stick around...with the entry of Nidhi, she tried every trick in the book to break them up...she used her friendship as a weapon...and when sh realized the futility of her efforts, she married Jagan the first time round and tried to kill Nidhi the second time...
This friendship had low points more than high!
Nidhi - Rohan- this was no longstanding friendship...they had known each other mere months...but yet got to know each other so well that they could second guess each other...Rohan wanted Nidhi's happiness at all cost...he stood by her side when she fought for Ashutosh...and yet, once she gave up the battle, he found himself falling in love with her...he was honest enough to admit the same to her...and man enough to accept her rejection...he stood by her side at the wedding and performed the role of a true friend...and then left her because he felt she had no need for him anymore and his presence might only complicate things..
On her part, Nidhi always cared for Rohan and was sensitive to his feelings...even when he confessed his love for her, she dealt it with in the most mature and sensitive manner...
Finally, she trusted him enough, even after that fracas during the engagement, to be by her side during the wedding ceremonies..and he fulfilled his role to the hilt...this is the most defining friendship moment in the show for me!