Why did Yaakov see fit to mention the fact that 'the G-d of his father was with him'?
Rashbam and Seforno: He meant to say that, in spite of Lavan's accusations that he had stolen his sheep, everything that he owned was what Hashem had given him, and that he had stolen nothing from Lavan.
Ohr ha'Chayim: Do not say that Lavan is understandably upset, for your flock is much greater than his. This was a Berachah from Hashem!
Malbim: Yaakov rejected possible reasons for Lavan's anger. (a) It is not because I sinned, and I lost Hashem's Hashgachah - "vEi'lokei Avi Hayah Imadi"! (b) It is not because I did not work properly for him - "b'Chol Kochi Avadti!" (31:6) (c) It is not because I took more than I am entitled to - he deviated and kept changing the terms! (d) It is not because I defeated him through the sticks - this was Hashgachah, Hashem did not let him do evil to me! (31:7).
Ha'amek Davar: He said Elokei Avi and not Elokei Avosai, for the Brachah of income is special to Yitzchak.
Oznayim la'Torah: It was clear that, since he had done Lavan no harm during the past six years, Lavan really hated him all along and that until now, it was only by the grace of the G-d of his father that he had concealed his hatred. But now that Hashem wanted him to return to his father's house, He had removed the mask of love from Lavan's face to reveal his true colors, in order to induce him to leave.