Here's what I use the various ammo types for, for whatever that's worth I guess (probably not a whole lot).
Cryo: rapidfire weapons, classes that can detonate cryo explosions (concussive shot, incinerate, warp, etc)
Disruptor: multi-shot pistols and sniper rifles (stun first, then headshot), classes with tech burst capabilities (overload, also other things that I can't be bothered to think of because overload is the most important)
Phasic: sniper rifles on characters built for sniping, and literally no other time (which means I have a huge stockpile of them for when I do feel like sniping)
AP: any hitscan weapon where I didn't put armor piercing in the mods because other things were more important or something
Drill: when I have more of these than AP on hand and want to save my AP for something else, or I just don't know what other ammo type to use and don't want to waste time thinking about it
Incendiary: powerful guns and the cheesiest of classes, and also anything with fire explosions
Explosive: I have no self-respect and don't even want to try, whatever, who cares
Warp: I almost never have these on hand because I use them immediately, but they're good for basically everything and especially useful for biotics
imo cryo ammo is pretty much useless on high-powered single-shot guns, for whatever that's worth. Those should be able to kill anything you might otherwise have frozen anyway. Unless you have TREMENDOUSLY bad aim and all your shots just end up hitting their feet or something, I guess. In which case maybe use a different gun, idk.