RealCalc Scientific Calculator is my favorite scientific calculator app for Android. The UI is beautiful and mimics most of the classic scientific calculators. The app is light and loads up quickly. There are a few settings that can be adjusted. This is a free app, but there is a Pro version with more customizations which is a paid app. However, the app is not naggy about the upgrade and does not show any ads either.
Tried with: RealCalc Scientific Calculator 2.0.3, Android 5.0.2 (Lollipop) and Moto G 2nd Gen (XT1068)