at first I thought it was s3 (peyton lost her mom, lucas lost keith, dan was the one who shot keith, the school shooting in general, breyton's friendship ending...again!, rachel's drunken car crash landing her nathan and cooper in the hospital, peyton realizing she still loves lucas, etc.). I guess those eppys hit me hard. but season 4 had alot more now that I think of it: The characters find out that dan shot keith, haley gets hit by a car, lucas' heart stops, karen nearly dies from delivering her child, psycho derek comes after peyton not once but twice, peyton finds out that brooke slept with nathan, haley and nathan have a baby, nathan loses his scholarships after he confessed to "point shaving". Yeah, I'm sure there's more, but the number of scenarios mentioned speak for themself. Both seasons were intense.

Season 3 and 4

omg def season 3, season 4 was the least dramatic

3 and 4 were the most dramatic, but they were the best seasons :)