What is the best voice changer device avaible?

Hello! and thank for answering my question. I would like to occasionally have fun with my co-workers using a voice changer. From what I have seen so far; the devices available requires me to use a computer or carry a complicated machine. I want to know if there is a device that is easy to carry and most importantly easy to use like if I want to sound like a old man; I would press a on and voila!!.. I sound like a old man. If there is no such easy to use device available; then ,in your opinion, which device would be easy to for a beginner. Thanks!