As above, chemistry would be most useful. Moreover, the "biological" aspect is inherently linked to the "chemical" aspect, as biological forensics tend to focus on molecular genetics approaches - which is chemistry based. With the exception to an extent of human osteology and related areas, the biology will very much be molecular biology based which is as above inherently chemical.
I would caution pursuing a degree in "forensic science" or similar, and advise to do a degree in a basic science (or related field of interest, such as possibly psychology or archaeology/anthropology - particularly biological/physical anthropology or human sciences, if you're interested in e.g. identification of human remains, more than DNA evidence etc) such as biology, biomedical science, chemistry, genetics, biochemistry, etc, etc. Such degrees may also offer options in forensic science/identification etc (particularly the biological anthropology/chemistry/anatomy type courses).
You would be equally, if not better, qualified for such positions with those degrees, and also qualfiied to do something else in the likely event you aren't able to immediately enter that field. As above, it's very competitive now - the popularity of police procedural shows led to the creation of such "CSI Degrees" whose combined student cohorts number in the thousands, compared to a few hundred positions in the country - which is actually all that is required, as it's not a high growth sector.