>>13819700Plutonium, specifically 238:
Gives off 570 watts per kg, half life 87.7 years, extremely small shielding requirements.
Also, 10kg critical mass, 10 cm sphere.
19.8 g/cc if you like density.
If I could have as much as I wanted, I'd make an airplane that flies for my entire lifetime. Use it to provide heat for a Brayton cycle engine.
