10's are an all around great performing speaker that can cover a wide variety of shows and hence the reason I like them more. They can offer high end extension up to around 5khz, which means you can roll off that horn tweeter if needed. They can offer low end extension down to 40hz which is enough to cover a 4 string, however most designs roll off around 50hz. I have never heard a 5 string player complain about their B if the cab is designed right.
12's are great performing speakers which tend to favor the low mids, have great punch, but an overall lack of upper frequency extension sitting between 10's and 12's. Generally due to the crossover frequencies that are used for most horn tweeters, this is a non issue.
15's have a rich, almost lush like sounding low end if properly designed. The smaller the cabinet though, the more likely you are to find a lack of lows, and a more pronounced overtone above the fundamental. They almost always tend to fall off in frequency response around 2k-3.5k. This is not a huge issue however if you use a lower crossed over high frequency device. 15's would have the greatest potential to sound best for bass guitar, but this is not always the case due to the cabinet size required for that kind of sound. They can benefit much from having a dedicated mid speaker such as a 6-8" cone.
18's are good for R&B, but will never have the highs for a full range tone. They will be able to produce the fundamental note of the E & B strings (or lower) however will require a huge cabinet to really open up. These cabs are great for a bi-amp design, and most players I have seen use them opted for a 4x10 for the tops. They rarely required ANY low eq boosting for a full rich tone.
I have no comments on other speaker sizes, as I have no experience with them.
My preference, 10's. They just fit right, and can offer the same low end extension that a 4 string player like me needs. 15's are my next preference, but I would opt to have a combination of 15's and 10's for my ultimate rig.