What's the best way to gain 10 lbs fast?

Assuming you mean muscle.

Calculate your TDEE, add protein until you are over by about 200 calories, lift weights.

Don't expect results over night.

I prefer snickers

If you want to maintain your health but still gain weight, the best thing to do would be to decrease your activity as much as possible. Don't tap your feet sitting at a desk, don't go out for runs, basically be as lazy as you can be and sleep alot. It would still take at LEAST 3 weeks for you to gain a full ten pounds, depending on your metabolism, but this is the fastest combined with healthiest way. If you just want to weight without regard to anything else, then do what I mentioned before, but add eating a whole lot of junk foods, plenty of salts and especially sugar. However you can still do this in a healthy way, you'd just have to increase the quantity of your diet.