Why did he say "Kirvas Elokim Li Tov"?
Radak: The Resha'im will perish because they are far from You (verse 27). Since I am close to You, I anticipate good reward in the world to come.
Mesilas Yesharim Perek 1: True perfection is only clinging to Hashem. Only this is good. Everything else that people think that it is good, it is folly.
Malbim: Since I am close to Elokim, I will find good and happiness.
What is the meaning of "Shati... Machsi"?
Radak: I rely on You to teach to me.
Malbim: I rely on Hashem to have special Hashgachah over me.
What are "Mal'achosecha"?
Rashi: They are Your Shelichus - Ru'ach ha'Kodesh in my heart, to say it.
Radak: Your deeds. May I understand nature, which is knowledge of all Your physical deeds below and above. Then I will know Your deeds, and Da'as Elyon!
Malbim: They are angelic, spiritual matters of the Nefesh, which is Hashem's Mal'ach (messenger) in the land to do His Avodah. It is like Elokim, to know what is good!