You're looking for episodes with nasty nature and science gone awry - at least, that's the impression I get. On that basis, I recommend these episodes from seasons 1-5:
S1: eps 3 and 21 - Squeeze and Tooms
Ice (ep 8)
Eve (ep 11)
Young at Heart (ep 16)
Darkness Falls (ep 20)
S2: The Host (ep 2)
Firewalker (ep 9)
Aubrey (ep 12)
Soft Light (ep 23)
S3: 2Shy (ep 6)
S4: Teliko (ep 3)
S5: Kill Switch (ep 11)
The Pine Bluff Variant (ep 18)
There are other episodes I could include, such as "Blood" from S2 or "Wetwired" from S3, but they also feature an element of government conspiracy which is more prominant in the story. "Squeeze"/"Tooms", "The Host", "2Shy" and "Teliko" all feature freakish genetic mutations, like Leonard Betts.