Only use ROMs that have been built for the Optimus One. And IIRC, there is one custom recovery for the OO.
There is no best ROM since the beauty of them is that they are customizable according to your needs. Ask yourself what you want / need from your phone and look at the different ROMs in the Android Development subforum. Choose one from there. There is also a beginner's guide somewhere here that could help you decide.
I'm using void because I dont like the many apps that were preloaded and it has the usual performance tweaks that most ROMs have plus overclocked a bit more that normal.