It's hard to pick one. I'll talk about college final year project. It was a research project in Analog VLSI.
Design of a high performance amplifier for brain computer interface systems.
The challenge lied mainly in the signals being used for the BCI systems, EEG signals. The signals being of the order of micro Volts and lie in the frequency range of 1-100Hz which is highly prone to noise and hard to detect precisely. Hence, there was a good need for a really effective amplifier that could amplify these EEG signals to a processable level with Very precise filters and low power cost.
I with my 2 teammates, discovered a way to efficiently redesign the existing amplifiers from the transistor level, resulting in very high gains with precise bandwidth that suited the specific application. There were many challenges and failures over and over. We were dealing with transistor widths and lengths in picometers. Small changes would result in seemingly impossible to explain results at first. We kept persisting, studied a lot about the techniques and processes and slowly started being able to explain the results and constructed our design based on the results. I am proud of that more than new technology projects like Blockchain and Machine learning is because, as a student, there was so much excitement whenever I learned a new thing, or got results for the hard work we did days and nights. Thank you.