For beginners, older and/or unfit; Lucy Wyndham Read. She is really lovely and gentil, and her workouts are very short, so you can do 2 or 3 to begin with and 'work your way up' to her Thighs & Glutes workout, which is VERY effective.
For intermediates, and those who are prepared to challenge themselves: there's LOADS, including Heather Robertson. Sydney Cummings, Rebecca Louise, Popsugar Fitness........
Also Growingannanas (more HIIT/cardio based), though some of her workouts also use dumbbells. Her workouts are very well structured. I like the fact that she doesn't talk. Some online trainers chatter constantly, which can be so annoying you have to turn the sound off. (I'm looking at you, Rebecca!)
But the very best is Caroline Girvan. She uses quite heavy dumbbells and her workouts are challenging - but very effective, rewarding and - for many - quite addictive. What I like is that she has several series which are incredibly professional and well thought out, targeting different body parts on different days.
I particularly like Caroline's EPIC Heat. Tip: Start slow and low, pace yourself, substitute as necessary if it's too tough. I've been doing her programmes for about 5 years and I'm now superfit. And I truly enjoy it. [NB: im 70...]