The poll, carried out by Ipsos Scotland for Channel 4 News, found almost one in three respondents (32 per cent) placed Ms Forbes as the best first minister of the three hopefuls.
Meanwhile, 24 per cent chose Humza Yousaf and 8 per cent picked Ash Regan, with a clear pattern by age, as younger people tend to favour Mr Yousaf and over-55s lean towards Ms Forbes.
However, among those who voted SNP at the 2021 Holyrood election, Ms Forbes and Mr Yousaf are neck-and-neck. One in three (33 per cent) said Mr Yousaf would make the best first minister, while 32 per cent said the same of Ms Forbes.
Overall, one in four said none of the prospective candidates would make the best first minister.
The findings came ahead of the Channel 4 News leadership debate on Thursday night. It is the second TV debate of the contest, with the first, held by STV, seeing brutal exchanges between the three candidates.
Ms Forbes sparked outrage from some SNP figures by trashing the record of her main rival, Mr Yousaf. She also suggested Nicola Sturgeon’s record in government was mediocre.
Ipsos polled 1,503 Scottish adults over the age of 16 between March 6 and 7 – before the STV debate was held. This included a subsample of 582 SNP voters at the 2021 Scottish Parliament elections.