So, many years ago I always just played whatever stock speaker came in my amp/cabinet, not really thinking that changing it out would make a significant difference. I had grown up playing crappy solid state amps with a closed back where the speaker wasn't even really accessible and others where it was hard wired so it wasn't much on my radar due to a lack of desire to take apart my amps/hack the external wires. This was also well before i started rewiring my own guitars and making pedals, etc. However, it really took a move to using heads over combos and a friend who was really into speaker swapping to make me realize jsut how huge a difference having decent speakers makes.
So, I played through a bunch of different speakers and got a feel for which ones I liked and didnt like. The first set that struck me as sounding really neat and different were a set of alnico eminence speakers from the 70s that have this thick aluminum dust cap (a lot of the thin aluminum dustcaps used later on, i've discovered, sound really ice picky and shrill). So, I kept trying out various kinds of speakers and not caring for a lot of them until I tried out a couple different types of made in England Celestions. So, now I am using a pair of G12H65s for higher wattage amps and thicker sounds and just got a pair of G12H75 greenbacks to compliment them as a somewhat brighter alternative. (The 75 stands for 75hz bass response rather than 65 watts as the 65 does in the previous speaker). HOLY CRAP! Perfect for my needs. I still also use those old alnico speakers for certain sounds too in a couple of extension cabinets.
I had assumed that the made in England stuff would be a bunch of bunko and on par with the made in china v30s and greenbacks. Well...nope. This turns out to be one case where spending some extra cash seems to actually be worth it. You can't do a lot to upgrade a speaker itself without entirely gutting it (unlike a guitar or amp) so, choose wisely friends.
So, what are your speakers of choice? Why do you like them?