Best study stimulants/nootropics: Caffeine, Betel, St. Johns wort and Ginko, either alone or in combination. All useful, But betel is definitely the best single study aid , not only giving long duration /stimulation, and light mood enhancement, but also substantially enhancing memory, especially good for exams.
More rarely, for acute time constraints, Methylphenidate, Adderal IR, Propylhexedrine extracts, as they often negatively impact my mood to some deg
Ideally though infrequently,a little (~200 mg tramadol or a medium dose Poppy seed tea) does increase my study duration, and enhances my focus quite a bit. Always makes the work that much more enjoyable and me more productive.