The Zohar Hakadosh
If there's one thing that Rav Shmueli repeats over and over in his shiurim, it's the importance of learning the Zohar Hakadosh. Written by R' Shimon bar Yohai, whose Yahrtzeit we celebrate on Lag La'omer, it is considered so important that learning it has the power to bring the Ge'ula with Rahamim. So says R' Shimon bar Yohai himself. (Watch from 45:49 until the end of Rav Shmueli's Shiur from Emor, where he says how it's especially true for our generation). See 'רעיא מהימנא דף קכד עמוד ב where it says: ובגין דעתידין ישראל למטעם מאילנא דחיי דאיהו האי ספר הזהר יפקון ביה מן גלותא ברחמי
One great Ma'amar from the Zohar that has everything in it is the Zohar Hadash in the beginning of Ki Tavo. It has the story of R' Shimon bar Yohai and the cave, an idea of turning the Tochaha of Devarim into consolation, and Keitz Mashiah all rolled up into one Ma'amar. (Please don't ask me to translate it for you. Please ask your LOR.) A very uplifting Ma'amar indeed.