With any workout, it depends on what your goals are and where you're at now. It also depends on how much time you are willing to commit to it and what resources you have.

I am breaking my own rules about not doing boring workouts by engaging in steady state cardio that is both a mental and physical grind to get through. Habit.

High intensity interval training wins my vote for all-round strength, cardio and efficiency in a workout. It's short, it's intense and it keeps you focused. With a trainer who is dedicated to your safety and fitness, it's also a great way to pick up technique and ideas for your own workouts. This requires no equipment usually - body weight cardio and strength are very effective when done properly. Classes that are based on this method include CrossFit, F45, Tabata and HIIT.
Try CrossFit Collingwood, F45 Training in Victoria, or talk to a personal trainer about designing one particularly to meet your training goals.

Circuit style training is a similar idea to HIIT, only you don't need to do the short bursts of really intense activity to such a strict time limit. That said, using your phone to time yourself in each activity or even designing your workout playlist around your circuit plan is perfect. I used to spend longer making my running playlist than I did actually running! Carl Cox got me through a lot of endurance distances. What might a circuit look like?
Cardio: Treadmill warmup 10 minutes
Strength: Wide stance push ups for chest; deadlifts and rows with a barbell or dumbells; clean and press
Cardio: Running up and down staircase for 2 minutes
Strength: Plie squats, plank hold (try the medicine ball plank pictured if you're up for a challenge!)
Cardio: Rowing machine or bike for 20 minutes
Try BodyPump, Barre Circuit or any type of hybrid class. My latest Fitness Network article is all about interesting and effective hybrid classes!

Whether your goal is weight loss, cardio fitness or building strength and definition, circuit style training and getting your heart rate up while combining cardio and strength work is going to meet your goals. It is the intensity and duration that you will need to tailor to your needs. Speak to a trainer with experience in working with clients that have similar goals to you.

Beware of overdoing it. Been there, done that. If you end up hating and dreading your workouts, and you're punishing and pushing your body, you will not only end up with injuries but even worse, a feeling of misery, exhaustion and failure. Work with a trainer who recognises your goals and provides realistic guidance and motivation. If you need help with diet and wellbeing, see a dietitian and/or a psychologist with experience working with sportspeople or others with your particular goals and health background.
There are free fact sheets and a list of accredited Sports Dietitians at SDA Australia.