Generally the loot from any monster that gives Level 3 Fame or above has a good chance to give Essence quality loot. As with any monster loot, it all depends on what you take, so you will still have to be selective, and take only EE quality loot. Even the toughest monsters can give you low intensity, single property items.
As for Fragment quality loot, you will want to look towards the toughest monsters (ML named, Paragons, Champions, Peerless, Doom Bosses) to give yourself the best chance to find Fragment quality loot. You will want a decently high luck, but even then there is no guarantee that you will find any.
Stayin Alive,
BG