What is the most risky form of investment?

When you place your hard earned money to others to "help you invest"

Anything without a stop loss or exit plan

IMO, the riskiest form of investment is the one you make without doing your own due diligence. I.e: listening to tips from friends/family/colleagues/etc and then investing straight without researching, etc

Definitely derivatives - CFDs and like what Mr Jonathan Chia has said, trading on margins. Since trading with derivatives leave you will no potential cap to losses, and the ability to tap on leverage to inflate your position you will see very huge swings in your Profit and Losses (PnL) when you trade these instruments. 1 bad trade leads to another emotional bet twice the size to recoup your losses, and you end up with huge amounts of losses that could be twice, thrice, five times your initial investment. always be careful with these derivatives!