לב טוב Lev Tov - a good heart.
This is one of the most vague and broadly interpreted lines in Pirkei Avos which leaves us bereft of the entire goal of R Yochanan Ben Zakkai. R'Y'B'Z' instructed his Talmidim to each find one specific trait to work on aiming for perfection so that many other middos will naturally follow suit. Giving a broad definition to לב טוב misses his point entirely. However R' Yonah gives a very specific definition; סבלנות - to perfect the trait of tolerance. Patience is just one form of tolerating discomfort. But learning to peacefully accept and bear any burden upon yourself is an exercise of ענוה as well.
We usually understand that being סובל difficulties and especially the wrongdoings and disrespect from others is a wonderful, but very specific trait. This R' Yonah opens up a whole new understanding that this middah is so great and all encompassing that all the others are included in it!