Best Ab exercise?

What would you guys say is the best ab exercise?

for most people running j/k.
 
for me hanging leg raises. 

Alright.  Im a pretty skinny guy and I don't have anywhere near a 6 pack, but my ribs are visible below my pecs, so I think my abs need work.
 
Ill try leg raises.

If you skinny you "BLESSED" you can get to see results faster than a fat man or woman.  For abs you have to do a weight training  program. Workon legs then abs the same day .....few realize this I actually do legs first after the squats then hit the abs then do the running for cardio ect.   Do legs first !!!! then Abs Second ...then a running type exercise third  because when you think about it aerobics =is  using mostly legs  so you do all legs and abs the same day 3 days a week . If you skinny and want a 6 or 8 pack then just add resistance to the ab exercise many people do crunches with just body weight ....in a warm up style....ect  but to build muscle you have to work muscle hard effort  but breif......so add weight resistance to the ab exercise if you skinny , trying to build up.

Ok.  There doesnt seem to be an ab exercise machine, but there is a chair type thing which I do vertical leg raises on.  Im probably going to put a weight on my lower body (Like a dumbbell) and do them that way.

Hanging leg lifts, "Frog kicks", are the best for a lower abdominal exercise. Crunches are good for upper abdominal, but i prefer full sit ups or sit up twists, which are good unless you have back problems. Waist twist are good for your obliques, so is side sit ups. 
 
To get a good trimmed abdomen you will need to combine these exercises, with a cardio plan (I prefer jogging since the twist helps tone the obliques even more) and a good diet.
 
You don't have to use weights (although it would probably be easier to get fast strength) to get a good abdomen, you can just use your body weight. Point and case; Bruce Lee had a rock hard abdomen and he only used his bodyweight.
 
I personaly do abdomen exercises every night with only my body weight
1 set of leg lifts
1 set of abdomen twist
1 set of sit up twist
if you use this method start with 10 of each, each night for a week and then add 5 every week after till you reach about 40, except for abdomen twist (your obliques should be able to do more). If you do get sore then take a break from it till you heal.

i've actually noticed good results in just about a month from this. I'll tell you what i do and you can do wtvr you want. I lift 4 days a week so i do this routine for abs 4 days a week. I do sit ups on an incline, but i put 15 lbs behind my head to make it harder, if you cant do weight then atleast do it with your hands behind your head. Do about 10 reps for 4 sets. Then you lie down on a flat bench so that your ass is just hanging off the end and you keep your legs as straight as possible for 30-45 seconds, so you don't fall of the bench hold yourself wit your arms behind yourself, its kinda hard to explain. Do that for 4 sets. Then last excerise, you sit on the end of the bench so your legs hang off, sit up right, and get a 25 or 35 or even 45 lb plate and do twists so bring it to your left side and lower it and hold it for 2 seconds and then bring it to the right side and lower it and hold for 2 seconds and just do that so you hit both sides 10 times, so you're pretty much counting to 40. That's it, i've tried many things but this works best for me.