The design I'm using is based around LTL logic by Tim. It has the following circuit diagram:
The noise marging should be quite good due to the difference in forward voltage between the input diodes and the led. As for speed, I've built the following 5 stage ring oscillator:
I don't own a oscilloscope (too expensive for my budget...), so I used a Arduino with the amazing freqcount library. The entire oscillator produced around 4 MHz when testing 2n2222a transistors, which is 20 MHz per gate and 5 MHz when testing the BC547C, which is 25MHz per gate. Other people have recommended me using the 2n3904, but I don't have access to the right now. I would probably use the BC547. It is worth noting that the capacitors add a big speed improvement.