Which is Most Beloved: Istighfar, Salawat, or General Dhikr?

As-salamu alaykum. I'm trying to understand which of these three brings the most joy to Allah and which one is best for getting my duas answered. I'm feeling a bit overwhelmed, and doing 1000 of each every day is really hard. I have to focus on my studies, so my time is limited. I know Sayyidul Istighfar and Durood-e-Ibrahimi are considered the best in their categories, but what about general dhikr? What's the best form of that? Please share what other acts of worship you do and how your duas have been accepted. (I'm aware of Tahajjud.) How do you structure your dua? I use the sandwich method-praise Allah, send salawat, then ask.