Not situps or crunches.Reason is already explained in another
older thread

Front plank(above)

Side plank(below)
Introduction:
Plank is a well-known exercise that we see in both Pilates and
yoga. It is one of the most popular exercises for developing core
strength and stability.
While Plank really targets the abdominals and shoulder stability,
you will find that plank is an excellent way to get a full body
challenge. In order to do Plank properly there must be integration
of all the core stabilization muscles. The arms, gleuts, and legs
are active as well.
Plank can look like the starting position of a regular push up.
But, in most cases, a regular push up entails much more strain in
the upper body -- especially in the shoulders and neck -- than
plank in Pilates or yoga.
You may want to begin with a modified version of Plank and work up
to the full version, especially if you are weak in the upper body
or have neck strain issues. Please see the plank prep
exercise.
You will know you're doing plank well when you have good form, feel
your center working, and have good shoulder stabilization yet are
not incredibly rigid.
Note: The name plank comes from yoga. Pilates has traditionally
referred to this move as front support, but that seems to be giving
way to the more commonly used term, plank.
http://pilates.about.com/od/pilatesmat/ss/Plank.htm
For videos on other core training methods/exercises
Search for "Optimumperformance" and "Captnj" on Youtube
respectively.