At first it seems like Phoebe is more of an assistant to Helga, however as the show progressed it was shown that they really did appreciate, care for, and needed each other. Plus the fact that Phoebe is the only one that knew Helga's big secret for the longest time was very endearing. Great example episode of their friendship would have to be "Phoebe Skips". I teared up at the end of that one.

pheobe skips is a good episode. Another episode that i liked where it showed their friendship was the one where pheobe breaks her leg and helga decides to make up for causing pheobe's accident by being her servant for a while until her leg is healed. I also liked the one where helga sleepwalks and she's afraid that arnold will know about her secret so she asks pheobe to stay over at her house for a while.

but i like arnold's and gerald's friendship because it's more deep and strong too.

anrnold and gerald cause helga sometimes uses pheobe which isn't true friendship

I agree with all of the above; both friendships are wonderful and interesting, but Helga's the one with the most to learn about how to treat your friends, so the Arnold/Gerald relationship is more balanced.