Noise floor is usually expressed as dBm, which is the noise power relative to 1 milliwatt into 50 ohms. Because the usual numbers are much lower than 1 milliwatt, the numbers are negative. A greater negative number is indicative of a more sensitive receiver.
Short Answer: More negative numbers are better.
Now for the caveat: Sensitivity of most modern receivers is so good that the noise floor of the receiver is much lower than the typical noise level coming from the antenna. This noise from the antenna can be 'real' noise, i.e. thermal or galactic noise; but much more likely in today's environments it is man-made noise from all the electronic crap nearby.
This means that having a receiver sensitivity of -133dBm doesn't really matter if the noise floor in your location is -103dBm, which is a typical number for 20 meters in a residential environment.
This also means that if you do not add attenuation on the low bands, you are wasting dynamic range of the receiver. One should almost always use 6-10db attenuation on low bands (80m or lower) unless you are using some sort of very low gain antenna such as a beverage or flag.
-Tony, K1KP