There are 9 assassins in the Dota 2 Auto Chess and they leap into enemy backlines. If there are 3 assasins at least, they would deal devastating critical hits to enemy. There are two different assassin combos which are given critical hits %350 and %450. In this case if you are lucky with your critical hits, your assassins will kill the units which are on the backlines before they use their ability.
Many people try to get orc combo with orc assassin at the beginning but, there is no any assassin as orc race :)
Bounty Hunter is the prefferred by the people who plays assassin build. Its upgrade cost is low and easy to keep the upgrading on board. This unit is the most popular part of opening strategies.
Morphling is the worst assassin because of his slow attacks. The players use it for the elemental combo to active his stun or completing to the three assassins on board.
Queen of Pain is preffer for next of the phantom assassin as a main damage till mid game. This girl should be on the back lines and need a tank to cover for the first fighting.
Viper is an important assassin for the other assassins. However the viper is ineffective as alone, If there is a viper with two or more assassins, viper could be like a supporter them.
Phantom Assassin is might be your killing machine with her critical hits. She deals very high critical damage to single target as an assassin. She is a good choice to try to get to three stars.
Templar Assassin even though is an assassin, you want to use her on the frontlines. She is a most survivable assassin with her refraction. Also she deals high damage like Phantom Assassin but without critical damage. Getting a three stars Templar is so hard.