This should probably actually be “my worst performance (out of many)”,.. I bought Faron a while ago at a price of €1.52. Now it is the best return in my investment history with a rise of about 120%.
But since the portfolio weight was 1% at the time of purchase and is now 2%, it stings a bit. Quite okay timing, but I lacked the courage.
Current top 3 buy & hold investments in percentages. There are other 2, 3, 4-baggers as well, and of course, a history of over 20 years includes multi-baggers that have already exited the portfolios. These top 3 buys were timed roughly at the short/medium-term bottoms of that time. I like to buy during macro crises and other special situations independent of the company, or during company-specific setbacks that I assess as temporary. Ideally, I aim to buy an “overweight” position, which I then try to trim on the way up. Sometimes it works and sometimes it doesn’t. Where was that “your worst investment” thread again? 
Novo Nordisk ~605%
Vaisala ~438%
Wärtsilä ~542%
Finally, a small recent anecdote: the Solteq case, which didn’t end well and thus doesn’t really belong on the list.
Solteq (was) +600%
In this case, I identified the potential; the entry and additions were made well before the rest of the market woke up. The exit simply failed completely for certain, partly even human, reasons. Only one trim on the way up, and the rest was left in the portfolio to rot. Now the company is struggling quite badly. My own losses have already been realized. At its peak, there was approx. €60k tied up in this. Fortunately, the portfolios (incl. mandates) are still over 600% in profit, so it doesn’t rock the boat, but it still stung.