Which major gedolim, especially at the times at which the events occurred, but in general too, have said Hallel for Yom Haatzmaut and Yom Yerushalyim?
The definition of a major gadol is, of course, highly subjective, and left up to the answerer.
Rabbi Eliezer Melamed rules in Peninei Halachah that one must say Halel.
He says elsewhere that Rabbi Shlomoh Goren (and Rabbi Gershoni, quoted in the footnote) ruled to say it with a blessing, and this is how Rabbi Tzvi Yehudah Kook acted. He also quotes Rabbi Meshulam Rata (Kol Mevaser 1:21) to say it with a blessing. Rabbi Sharki (quoted in Sidur Beis Meluchah), Shu"t Netzer Mata'ai (36), and Rabbi Ch. D. HaLevi (Das HaMedinah p. 82) ruled to say it with a blessing. Rabbi Y. L. HaKohen Maimon ruled likewise in his own synagogue.
However, he quotes Rabbi Ovadyah Heda'yah and Rabbi Ovadyah Yosef (Yabia Omer Orach Chayim 6:41) who ruled to say it without a blessing. Rabbi Yosef Mashash (Otzar HaMichtavim 3:1769) ruled to say it with a blessing; however, he says that when Rabbi Mashash heard Rabbi Yosef's ruling, he ruled that someone who had the practice already not to bless should not bless (Shemesh UMagen 3:63,66). Rabbi Sha'ul Yisra'eli ruled not to bless. This is also the opinion of Rabbi Avraham Shapira and Rabbi Mordechai Eliyahu (quoted in HaRabbanus HaRei'shis vol. 1 pp. 901-903).
Rabbi Ushpiza'i, Rabbi Hertzog, Rabbi Uzi'el (brought in HaRabbanus HaRei'shis vol. 2 p. 841), Rabbi S. Y. Zevin (quoted there p. 890 note 6), and "HaRav HaNazir" (quoted by his son) all say that when Israel was first created they should have ruled to say Halel with a blessing; but since they didn't, we can no longer say it with a blessing.
Rabbi Yosef Dov Soloveitchik (quoted in Nefesh HaRav p. 97 quoted on this website) ruled to say it without a blessing. Though Fred quotes a student of his who said he would cover himself so no one could figure out what he was doing.
After I searched a little bit, I came up with these other sources not quoted by Rabbi Melamed:
Rabbi Yehudah Herzl Henkin (Bnei Vanim 1:16 and vol. 2 p. 210 in the addendum about his father) quotes his father Rabbi Yosef Eliyahu Henkin who said not to say Halel at all, but said not to stop those who did say it because they have on whom to rely.
The Minchas Yitzchak (10:10) is against saying Halel on "ימי חגם אשר בדו מלבם."
The Lubavitcher Rebbi (Igros Kodesh 9573) also says not to say it.