This is honestly hard as hell! The more I think it over, the more I realize I like them all to some degree. I can't settle on one, but I will say...
I'm seriously tempted to go with Tsukasa, even if it's still too early to tell how he'll turn out at the end. They could make him a slam dunk, or completely drop the ball. But he's got two things going for him right now:
Inoue's performance, which I find strangely fascinating to watch. The leads are usually interesting anyway, but this is a special case. Either he's getting really good direction, or he's putting a lot of thought into how he plays certain scenes. #3's still got the stand-out moment for me, but there's been a lot of good ones. I've no clue what he's like in anything else, but he's great in Decade.
The other thing is that I'm really interested in finding out who the hell Tsukasa actually is. I love the idea of giving a fairly straightforward hero a deeper background, which comes out in layers. There's something lying beneath. He's still unquestionably the hero, but there is a complexity to him. He could even be the ultimate bad guy!
Most Kamen Riders have this by the bucketful, but Tsukasa's especially cool since he also doesn't fit in anywhere. He's an anomaly and an enigma. I like what they're doing with this one.