I keep things pretty simple.
I like to inject my pork shoulders with apple juice and BBQ rub.
For chicken...I don't usually inject. Instead I marinate or brine it in a basic solution of water, sugar & salt/BBQ rub. I also like to marinate chicken in Italian dressing and occasionally some of that cajun/creole butter stuff.
Beef (if/when I inject) usually gets a watered down solution of worshy sauce mixed with some spices (onion powder, garlic powder, etc.).