My short answer would be Tren, but there are numerous factors that need to be taken into account when deciding which steroids are best for a person at a particular time, etc.

Tren. keeps strength and lean muscle mass while you diet.

Tren is the only steroid I know of that actually aids in fat loss all by itself. Not to mention its best freinds with the AR, works well at suppressing shbg and cortisol for a while when introduced. Used during dieting OR bulking its an excellent choice.