One of the two smartest races in the Mass Effect galaxy, only second to the Salarians. This technological genius race may have weakened immune systems but they have a frightening capacity to repair their ships on spare parts alone as well as a knack for creating synthetic artificial intelligence.
While they aren't particularly hostile, it's one race that definitely is advanced way beyond our comprehension. Maybe they could explain what those crop circles mean?
The Quarian are not an overly violent group, more of a governmental race. However, if a strong willed individual comes into the picture, they can be rallied to war with all the tech they have amassed over the years.
Something to fear in terms of fleet size (50,0000 with 17 Million Quarians).