In general, experience has shown that most of CASIO's loudest watch models have been vintage models (made 10 years ago or more), with some not even being G-Shocks. Thankfully a few more recent models have shown reasonably good loudness (like the G-7710). Still, it's rather sad that CASIO hasn't been able to be consistent in this area, as some newer models have demonstrated seriously weak alarm tone decibel levels. The general rule of thumb is that solar models will be weaker than non-solar ones, as the operating voltage is lower (because of the nature of the battery chemistry, for trickle charge characteristics).
What I do find a little peculiar is how some G-Shocks have very decent loudness for countdown timer warnings, but then have a fainter tone for the basic alarm. Some examples are the G-7800 and GW-2500. I guess in this case, you could set the CDT to go off about the time you want to wake up.
So, I took a long survey of information from various places and decided to compile a list for the convenience of other WUS members. The choices I've selected are based on a myriad of different postings from across this forum, some of them established from actual decibel tests conducted by WUS members and others from weighted general opinion. I do not consider this definitive, but it can serve to give a reasonably good idea of what CASIO watches will have fairly decent alarms. The benchmark watch I am using is the DW-5600E, which has a relatively acceptable alarm level for a small digital watch. Perhaps not enough to wake up folks with average hearing, but acceptable as far as a reminder/notifier tool. Thus, the watches I've listed are reported to be louder than the DW-5600E. I've listed them in a descending order of loudness (some adjacent to each other may be identical), which is fairly correct for the first few models and then trails off to somewhat more subjective ordering. If you do see ordering that you feel is definitely wrong, or a model missing that should belong here, let me know and I'll try to make corrections as long as the forum allows.
NOTE: I've also added watches with vibration alarm near the bottom, as this may be useful to those who find no CASIO watches loud enough (due to reduced hearing sensitivity, inability to wake up to any watch tones, or a loud environment that drowns out most digital watch beep tones).
G-Shocks with reasonably loud alarm tone (greater than DW-5600E)
GM-100 (very unique mix of loud melodies)
G-77XX (e.g. G-7710)
G-30XX (e.g. G-3010)
G-7800 (louder CDT warning than alarm)
BA-80 (link) -- definitely loud! Just look at that impressive megaphone attachment.
SGW-200 (also SGW-100, SGW-300)
TW-7000 (548 module)
CASIO watches with Vibration Alarm
Some interesting reads on the subject:
Bleeps, Sweeps, Creeps
G-Shock Loud Alarm