A combination of L-theanine (200 mg, 3x/day) and mucuna extract (NOW brand, take 3-4 times the recommend dose 3-4 times per day).
Fat burners are mostly useless, a 5% increase in lipolysis (and that's being generous) from a "fat burner" means that you essentially burn an extra 100-200 fat calories per day. It would take 17 days to see a loss of one additional pound, given diet stays completely on point. Even *massive* doses of T3, at most, result in only about a 35% increase in BMR (1), and this is with a corresponding increase in appetite, muscle catabolism, elevated SHBG, hypoglycemia, and cardiovascular side-effects.
Appetite suppression is the most important factor in any weight loss drug or supplement, and the combination of the two that I listed above are superb for this, without any overstimulation.
TL;DR: Fat burners are mostly a scam. The majority of the weight loss effect is due to appetite suppression.
1. J Physiol. 1997 Dec 1;505 ( Pt 2):529-38.