Weather Ball isn't just a good attack - it's one of the most powerful elemental attacks in the game:
Base 50 x2 for any weather x1.5 for STAB (except in Sandstorm)
If the weather is sunny or rain, you also get x1.5 for a fire or water attack in that weather type = 225 power fire or water attack. The ice version is only base 150, but that still makes it the most powerful Ice attack in the game (plus hail damage vs most targets).
Thunder and Solarbeam are available to Castform; the set I use is this:
Weather Ball
Thunder
Rain Dance
Hail
The "hot" version of this would be:
Weather Ball
Solarbeam
Sunny Day
Rain Dance/Hail
You really need to support Weather Ball with at least two weather moves (which could be provided by teammates I guess), and relying on only one damage attack is not usually good (especially one with only 10PP). You could take both Thunder & Solarbeam and one weather option, I guess.
It's hard to see other sets being worthwhile.