What are the implications of "asher Yih'yeh ba'Yamim ha'Heim"?.*
Rashi: It implies that one should bring the Bikurim to him - even though he may not be as great as his predecessors. 1
Ramban #1: It implies that the Bikurim must be given to the Kohanim of the Mishmar that is serving in the Beis-Hamikdash at that time. 2
Ramban #2: It implies that if one gives one's Bikurim - or his Korban to be sacrificed - to a Kohen who later turns out to be Pasul (a Ben Gerushah), his Bikurim and his Korban are Kasher. 3
Seforno: It implies that one should afford him the esteem that is normally reserved for kings and prophets - by using the expression "Hashem Elokecha", since he is standing in for Hakadosh-Baruch-Hu, the Owner of your land, to whom you are bringing your Bikurim.
Targum Yonasan: What the Pasuk means is that only brings Bikurim to the Beis-Hamikdash if there is a Kohen Gadol serving. 4
What is the point of the declaration to the Kohen?
Rashi: To explain to him that he (the owner) is not ungrateful for the kindnesses that Hashem has performed on his behalf. 1
Ramban and Targum Yonasan: It is a declaration of thanks to Hashem (who promised the Avos that He would give us Eretz Yisrael, 2 and who kept His word - Ramban).
Seforno: By bringing his Bikurim to the Beis-Hamikdash, he is making a statement to all and sundry 3 (that he is grateful to Hashem for his kindness).
Why does the Torah insert the word "ha'Yom"?
Rashi: It implies that the Mitzvah of Bikurim - both the actual bringing and the declaration 1 - applies only once a year.
See Torah Temimah, note 15, citing the G'ro.
What are the connotations of "la'Hashem Elokecha"
Ramban: It means that the owner is bringing the Bikurim in the Name of Hashem.
Seforno: Refer to 26:3:1:4.
What are the connotations of the end of the Pasuk "Ki Ba'asi el ha'Aretz ... '?
Ramban: 'Since Hashem brought me to Him to serve Him in the land'.
Seforno: 'I came from another country to this land as a stranger, and am bringing the first-fruit to Hashem, the Owner of the land, who granted me residence - as befits someone who receives a piece of land as a gift or as a share-cropper'.