I'm looking forward to Leto's take on Joker, but the idea that it'll be better than Ledger's is a little bit silly, when you think long and hard about it. Leto is playing the character in a full blown comic adaptation with all the inherent trappings of that kind of movie. It will be big, bold and crazy, and will probably do a decent job of depicting the broad strokes, larger than life comic book villain we all know and love. It's likely to be good - but not all that subtle.
Ledger played a ****ing psychopath. A psychopath in a grounded universe, directed by a man who knows how to draw out characterisations in an expert fashion. It was an utterly believable performance. The Joker in the real world.
Go watch TDK again. Watch Ledger's speech patterns, his physical tics, his eyes... Think about just how much he disappeared into that role, and was 100% convincing in every second he was on screen. It's one of the greatest villain performances in history, in any movie, in any genre. It's a cultural touchstone.
Leto will be Jared Leto in a cool costume playing a tattooed laughing lunatic. And that's fine, but it won't compare to what Ledger did IMO. That was a once in a generation performance.