L-sits are a good basic abdominal exercise. I'm not sure it's the most effective but it is very important.
Brandon covered other exercises such as
lying leg lifts, hanging leg lifts (all kinds such as leg lifts from L-hang to candle hold), hanging stalder leg lifts
body levers, hang to inverted hang pull through front lever,
skin the cats (tuck/pike/straight)
L-sit lifts to V or manna, L-sit walks.
Inverted situps (GHD situps), bent/straight leg situps (weighted)