"I lost the hang of it" is best. It is most natural-sounding, and it captures the idea of having once been good but now you are no longer so good. "I became horrible at this" is OK, but sounds a little strange to me (I can't quite say why). "I got rusty with this" is OK, but would be a little better as "I got rusty at this." Other possibilities: "I'm not what I used to be (when it comes to this)". "My skills have really declined/eroded." "I'm past my peak" (if the reason you now suck is that you are getting older).

Your first and third ideas sound the most natural. The way I would say it would be "I've become rusty at this." However, the third of your suggestions is also very common. The middle one is technically correct; however, it would only generally be used if someone had become very bad at a skill they used to have. Not something that would never come up, it's just something that would be less common.

'I'm a bit rusty' or 'I lost the hang of it' are probably the most popular. John had some other good ideas as well. 'I became horrible at this' is not something a native would say!