The Mishnah says (Uktzim 3:12): ‘Rav Shimon ben Chalafta says: Hashem did not find a vessel that can hold blessings for Yisroel besides for peace, as is stated (Tehillim 29:11): ‘Hashem gives strength to His nation. Hashem blesses His nation with peace.’’
Sefer Shaarei Chaim (Parshas Pinchos) explains that all other brachos are only considered to be a success if one receives them in abundance. For example, money is only considered a successful blessing if one gets a lot of it. One who has a little money is not considered successful. Only someone who has a lot of money is considered a success. When it comes to shalom, however, even if one has very little it is still a success, as the pasuk says (Mishlei 17:1): ‘When one has peace, stale bread tastes good.’ Even if one has ‘empty vessels’, meaning he has nothing, shalom is enough to fill them up and provide all manners of success in and of itself.