The best enlarging lens is any high-grade lens SPECIFICALLY DESIGNED for the intended format and used within the manufacturer’s specified magnification range.
The shorter the focal length, the greater the magnification for a given projection distance. While 105 mm and 100 mm enlarging lenses are often cited, there are shorter, higher-magnification lenses specifically designed for the 6 x 9 cm format as well.
One example is the 80 mm f/4 Rodagon WA.
Here’s an idea of the magnification differences. Suppose you have a maximum distance of 1000 mm from negative to the print with a particular enlarger. That gives the following combination of focal length, magnification, and projection size assuming a 56 mm x 82.6 mm negative (say, Fuji 690). The following calculated results ignore the nodal distances, which generally make only a small difference in the outcomes.
f = 80 mm, 10.4X, 582.6 mm x 859 mm (22.9” x 33.8”)
f = 100 mm, 7.87X, 440.9 mm x 703.8 mm (17.4” x 27.7”)
f = 105 mm, 7.39X, 413.75 mm x 610.3 mm (16.3” x 24”)
f = 135 mm, 5.21X, 292.0 mm x 429.8 mm (11.5” x 16.9”)
I use a 105 mm f/5.6N EL Nikkor for the 6 x 9 cm format and find the results excellent.
A standard-design 90 mm enlarging lens doesn’t see and project a sufficiently wide angle for the 6 x 9 cm format, particularly at large magnifications (which requires moving the lens relatively closer to the negative). I don’t know of any 90 mm wide-angle enlarging lenses.