what in the opinion of the forum, is the best way to diffuse co2 within, say a 300 ltr system?

Well, you can use CO2 diffusers and mist, a venturi, or a reactor.
Or a combo of them.

Sort of up to you really.

Regards,
Tom Barr

I can stick a diffuser under the intake of a canister filter and have it sucked up into the filter, IF the filter can handle it. Lots of ways really. A lot depends on what you like.

Misting, etc will have little Co2 bubbles blowing around. Not a big deal in a tank with a clear background. Black backgrounds seem to make the bubbles really stand out. Some people think they are pretty, others think they are distracting. Using external reactors, etc will keep most of the bubbles out of the water column. Placing a diffuser under a powerhead can cause extra noise as the bubbles are chopped. Again, all the methods work, just depends on what kind of setup works best for you. That's the wonderful thing about Co2 enrichment, there are many versatile methods and they can be tailored to suit your personality and needs.