What is the most effective vitamins?  

vitamin c,glucosamine,magnesium,selenium supplements...if you want to gain weight eat 500 more calories than what you usually eat...drink lots of water...one must do intense physical activites so the body demands more water..try a small amount of alcohol (4 ounces of wine, 6 ounces of beer, or a half-ounce of liqor with juice)

of course vitamin -c specially natural like herbal medicine,
and calamansi juice

Eat more.
That's how to gain weight easily.

Vitamins that helps absorbing nutrients necessary for weight gain are the B vitamins. But to make sure you also not deficient of other vitamins, and to keep your body stronger, and functions better, just take multivitamins. And make sure you have all the B vitamins in it.
As I remember, there are six vitamins under the B vitamins, B1 to B6. They are all necessary for nutrients absorption and body functions.

Good luck!

Well take more badam soaked in water the previous nite and eat them the next day.

Do you want some shortcuts to gaining weight? Vitamin supplements can help, but if you want it fast, I suggest you eat foods rich in carbohydrates and fats, then drink a lot of bird, almost everyday and every night. Sure, you'll be a heavy weight in no time!