Honest pick: the IoT protocols project from my exchange semester in Sweden, 2024-2025. An HTTP/REST server, a CoAP client, and an MQTT broker, all in Rust, with the course rule that no external libraries were allowed. So no tokio doing the heavy lifting for you, no pre-built protocol crates. You read the RFCs and build the thing yourself.

Why it wins for me:

• It's the only project where I couldn't lean on an ecosystem. Most web work is choosing good libraries and gluing them well (which I enjoy, and which my day job at Capia mostly is). This was the opposite: raw sockets, parsing, and figuring out why your broker deadlocks.
• MQTT especially. Publish/subscribe, QoS levels, keep-alives, retained messages. Implementing it badly teaches you more than using it correctly ever will.
• It's also the furthest from what I do daily, which made it feel like actually learning something new rather than reapplying patterns.

Honorable mention: at Capia I built an observability setup with OpenTelemetry across 8 repos into self-hosted SigNoz, so any claim about performance can be backed by a trace from the frontend call down to the DB query. Less fun than a Rust broker, more useful.

If you want, the IoT repo has the code, or I can talk through how the MQTT broker handled connections.