Megid is good for grouped up enemies, does great damage. It struggles sometimes in groups because of how slow it travels, your team may kill the things before megid even reaches them. It's got decent power though. It's not bad on single target either, and is a good fallback if you're having trouble hitting with Gimegid or Samegid.
Gimegid is your go-to nuke, but doesn't work if the target moves. For stationary targets though, it does great damage.
Samegid can be good for single target, especially when the enemy is moving too much to use Gimegid. You need to get used to the aiming though, because it can miss with 2/3 of the projectiles if you're too close.
The rest are pretty useless. Ramegid [Type 0] is supposed to be good, but we don't have tech crafting yet on NA so can't use it. Sometimes I switch to Ilmegid when I'm in a group and my team is killing everything before Megid can reach them; it's damage leaves much to be desired and it kills your PP, but can work on easier content and is pretty easy to use.