Best bronze card in the meta?

You can judge a card's value by its expected point efficiency. Gwent cards generally play for a floor of ~100% provision value (or less if the potential upside is significant) up to a ceiling of ~150% provision value if some conditions are met (cards with stronger constraints can go up to 200%). Most cards end up having an efficiency in the range of 100-150%.

IMO, BlightMaker (with the mage combo) is the strongest bronze in the meta. This bad boy is an 11 point play for 6 provisions, giving him a ceiling efficiency of 183%. The constraints to achieve the ceiling are: 1) you also include Mage Assassin, 2) you still have Mage Assassins in your deck and 3) the enemy has at least one unit in play. 1) is insignificant since most decks need some filler bronzes. 2) is a small constraint, we estimate you miss Mage Assassins ~5% of the time. 3) is a rare problem (occurs maybe if you are up-against some no-unit deck or the damage hits armor, let's say 5%), but even then you still land 9 points (150% efficiency).

However, there is some additional value to BlightMaker: 4) incidental removal, 5) deck thinning and 6) tutoring. 4) upside is you may remove a weak engine with 1-2 points (but is so unlikely we will ignore it). 5) has some actual value: thinning improves consistency and we value it half a point. 6) is rarely used, but deliberately missing the Mage can be valuable in some scenarios. If you have card draw in your hand, you can tutor (e.g.) Yenvo to the top and play it two turns later to yeet the opponent's bomb (or guarantee you draw Yenvo next round). Tutor+play in Gwent is valued at ~8 points, but BlightMaker only tutors to the topdeck, making it trickier to estimate that point value (let's say 2 points). The cow has only incidental poison utility (which we will ignore), and so in the case of 2) and 6) you play for an estimated 6 points (100% efficiency).

In the end, you play BlightMaker for 6 points 5% of the time, 11.5 points ~90% of the time and 9.5 points ~5% of the time, giving him an expected point efficiency of 183% on the turn he drops (!). Which bronze do you think is the strongest at the moment?