He was just as strong or even stronger, as he was more reliant on his Force powers yet still a formidable fighter. He also had robotic legs which gave him several advantages, and he manhandled his brother Savage Oppress with relative ease when Savage challenged him, even though Savage was able to hold his own against several Jedi as well as Ventress and Dooku (albeit with circumstantial help).

The main difference is that Maul is much more cautious and calculating than before, as he is trying to think less like a Sith Assassin and more like an actual Sith Lord, with somewhat mixed results as his training as a Sith Lord was sabotaged by Sidious who preferred him as an agent. TPM Maul was still a very intelligent fighter and knew how to control the engagement, but TCW Maul put much more effort wherever possible into arranging ambushes and controlling the field of engagement than TPM Maul did, mostly because TPM Maul got chopped in half for that recklessness.

TCW Maul also has the glaring disadvantage that he had gone feral and was living in a trash heap for the years between his “death” and revival, so while his lightsaber skills seemed not to have diminished much (he was still able to fight evenly with a vastly improved Obi-Wan Kenobi, who beat him first time due to luck as much as anything else) he hasn’t really developed any new skills or knowledge either. It’s a testament to how good he was before that this doesn’t stop him from becoming a serious thorn in the side of the galaxy during the Clone Wars, but he still manages to be more of a painful annoyance rather than a real threat in the grand scheme of things.

Rebels Maul is a bit different- he is older and he has clearly put more effort and time into researching the Dark Side, Sith lore and Dathomir magic than before, although he doesn’t seem to show any new combative skills. It’s hard to tell if his skills have waned as, to be frank, everyone looks weaker in this show than before (except Kenobi) as far as lightsaber combat goes due to stylistic changes, but he’s still a skilled fighter even if he seems a bit less acrobatic (just a bit, mind). He does seem a bit thinner and less “ripped” than before, but this is Maul at his most scheming and cerebral, for whatever that is worth. At the same time, by the end of the show he seems to have a death wish, even if only a subconscious one, so that needs to be taken into account as well.

Honestly, all three might be evenly matched. It’s less that he grew or diminished in strength, and more that he changed styles over the years, meaning his evolution seems more horizontal than vertical. If all three of them were to fight, it might be a toss up who would come out on top.