I agree with Gary. "Eight point four three" sounds most natural to me. (Also, I don't agree with Wisteria's comment. I'm in the US, and we do read "point" when there's a decimal. In some cases, where the context makes it obvious that there's a decimal in the number, you don't have to say "point," but people usually do say it, just to be clear.)

I would say it the first way, and the second sounds odd to me, but I would understand it. This may vary between different flavours of English.

'forty-three' can only exist on the left-hand side of the decimal point. There, there are units, tens, hundreds, thousands, etc. They can't exist on the right-hand side of the decimal point, so everything after the point is a single number. 43.43 (forty-three point four three) 17.82159 (seventeen point eight two one five nine)

I'm going to be scattering my answer across comments so I decided to answer this one because I thought it was interesting. In the context of money, we do not say the point: $1.43 becomes "one forty three". It could also be many other things, like "a buck forty three", et cetera. In the context of anything else, we would say the point: "one point four three" and "one point forty three" are both acceptable. There may be regionality in this, but if the number of digits after the point is only two, it is very common for me to hear and say the composite "one point forty three". If it is more than two, this will never be done.

有个土办法：你想想，假如后面有更多的位数呢？比如8.4321，总不能读成 八点儿四千三百二十一 吧？所以还是读成八点儿四三比较 好 ：） 其实读成 八点儿四十三的，我也听到过，国外广播里，不过很少。