Plasma Grenades for:

Grunts; when it sticks to them, they will usually run into their comrades asking to take it out (which is useful for killing great masses!)

Elite; will usually dodge them but if you get close enough you should try to stick it on them (should kill em' first time, if not they get pissed off!)

Hunters; they are slow at dodging em', but its useless (only if you want an angry Hunter charging you)

Flood; don't try it, if it sticks to them they might jump at you and bring you to your death. It is effective against small ones, but only if it doesn't stick to them.

Jackals; they usually ignore grenades.

Now, Frag grenades for:

Floods; These won't stick to them which is good against the small ones.

Grunts; Useful when cornered, but not very useful when in open fields (maybe against large packs otherwise they can easily dodge them)

Elite; Again, one should do the trick when cornered but in open fields its no good (they dodge them easily)

Hunters; Useful anywhere (they are slow at dodging them) but it will only get them angry instead of killing them.

Jackals; will usually ignore grenades, but be careful with frag grenades: they might bounce off their shield and come back at you.

Another tip when you need to kill great masses of aliens or in multiplayer games:

When throwing a plasma grenade on the floor, their is a 3secs pause before it blows up. Now, when wanting to spread grenades, throw a plasma grenade anywhere on the floor (preferably near your opponents) then quickly throw another right next to it. When the first grenade will explode, it'll catapult far away any other grenade near the explosion.

Hope this will help you. Have Fun!