Star Trek is populated with many different alien species. Some encounters it seems like you better just pray that the kill setting on your phaser is strong enough to zorch the life out of them before they can melt your face off. Others you can reason your way out by using diplomacy or good times in the holodeck. Here's a list of some memorable, or not so memorable races that are ranked on that continuum.
http://www.ranker.com/list/hostile-races-in-the-star-trek-universe/switchdoctor,
Klingon
e.g. Captain Koloth, Duras Sisters, Worf.
Gorn
e.g. Gorn Captain, Slavemaster Slar
Cardassian
e.g. Gul Dukat
Romulan
e.g. Nero, Admiral Valdore
Bajoran
e.g. Ro Laren, Kira Nerys
Species 8472
Vulcan
e.g. Spock, T'Pol, Valeris, Tuvok
Borg
e.g. Third of Five (Hugh), Seven of Nine, Borg Queen, Crosis.
Tholian
e.g. Commander Loskene
Augment
e.g. Khan Singh, Malik