I've always been more of a Warioware fan than a Wario Land fan but I love the biker fit for Wario. I know this is purely headcanon but I've always seen it as Wario seperating himself from Mario's image out of spite. Becoming something of his own...
Really can't go wrong with Wario's classic getup, but I'll admit I have a huge fondness for the Ware biker outfit and how it makes Wario more distinctive from Mario, while also really fitting his bad boy persona. I really wish more Mario media...
I like Shake King because he's got a lot of parallels to Wario, and indeed makes a great rival, but he's not a rival in the same way Syrup is to Wario. Syrup and Wario are both equally as crazy about the money and are sometimes villians too, but...
I also agree on the Shake King being a fun parallel to Wario; he feels like a Wario who never moved on from his more villainous ambitions. A particular detail I love (though I'm not 100% sure is intentional) is that you encounter the Shake King...
I really like W:MoD's thief Wario outfit and I wish it was seen more. I just like the design a lot. But I guess my favorite would be classic because of Wario Land.