If I had to choose only one exercise for triceps I would go with Triceps Pushdowns. This exercise has always been a staple in my triceps workout. You can't achieve greater contraction of the triceps with any other exercise as you can with Triceps Pushdowns. Plus, what I like about it is that you can use different attachments to slightly work different areas of the triceps. You can use the straight bar to target more the long (inner) head of the triceps or you can use the V bar to target more the short (outer) head of the triceps. I often use the rope as well as a finishing movement for the triceps to get an extra squeeze and bring more blood to the area.