I think this answer is very personalized. I heard Rav Ephraim Becker teach that Mussar is about three things: where you are, where Hashem wants you to be, and how to get from here to there. Because we each have different middos, different challenges and different proclivities, the "best book" will be unique for each person. And unique for each station in life; the ideal seifer for you today may not be the ideal seifer 10 years from now.
So really, the only good answer I could give you would require at the least chatting on the phone. If you're interested, email micha <at> aishdas.org and I will share my number.
Right now, I live with Chovos haLavavos in my pocket. (With a larger one with commentaries on my desk.)
Others mention in their answers the centrality of Mesilas Yesharim. I tend to be cerebral; more inclined to study about a Middah than to study to internalize it, to make changes. Easy way to feel inspired without doing the real work. So I found the Ramchal's preliminary version of Mesilas Yesharim of more value. Unlike the final version, it is written as a dialog between two old friend, a Chakaham and a Chassid. The Chakham intellectualizes too much, to the Chassid's words about taking everything to heart, in fact the whole format, gives a message I need to hear.
But then there is another question of customization... Mesilas Yesharim as a whole is a ladder up to G-d. If you relate to Judaism in terms of finding a relationship and connection to G-d, his -- really Rav Pinechas ben Ya'ir's -- list of middos would be central. But if you are more inclined to learning how to emulate G-d, to refine one's middos so one could partner with Him to bring His good to other people, maybe Tomer Devorah (from a Qabbalah perspective, but potentially very pragmatic) the sample middos in Cheshbon haNefesh (a more rationalist angle) would be more appropriate for you.
If your goal is to keep your mind on the end game, the big picture, the point of it all, and you are of the "emulate G-d" sort, I might recommend R Dessler's Qunterus haChessed. (Found mostly in Michtav meiEliyahu vol. 1, but he has addenda later.)
And of course I have to recommend R Shimon Shkop's introduction to Shaarei Yosher, having spent so many hours teaching it and then writing a seifer that develops a whole worldview from it. I made the introduction with translation available online here.
But in Ohr Yisrael, Rav Yisrael Salanter writes about not squandering a moment of inspiration. You can get into planning and finding the perfect fit, and by the time you get started the inspiration dissipates and nothing gets done. So, I would start with Rav Shelomo Wolbe zt"l's (Alei Shur, vol II, sec. 1, ch. 14 pg 175, item #6) list of core mussar sefarim:
The essential mussar books, that which is called “studying mussar” are: Chovos haLvavos, Igeres haRamba”n, Sha’arei Teshuvah, Orchos Tzadiqim,Tomer Devorah, Mesilas Yesharim, and Or Yisra’el.
Youths, up to the age of 16, who have no purpose for the books listed above, could learn Tana deVei Eliyahu, Mishlei with Rabbeinu Yonah’s commentary, Tr. Avos with Rabbeinu Yonah’s or the pious Ya’ave”tz’s commentary; or of the acharonim, Seifer Shemiras haLashon of the Chafeitz Chaim, or Michtav mei’Eliyahu col I by R’ EE Dessler.