Tough question, IS300 came with a pile of suspension mods so it comes close handling wise, the G450h was a stunning in mid range, the IS250 was a stopgap as my GS450h was 4 years old and the new model GS was due the following year then a year later the new IS, didn't really get on with the IS250 as I'd got spoilt by the GS. IS200's had a fantastic gearbox and engine noise when you ragged them, could have done with 20-50bhp more, hence the IS300 later on.
So bloody glad to go back to a CVT/Hybrid again, would still like the GS450h power in the IS300h body thou, but I think they have all got pluses and minuses, even the IS300h isn't perfect, but it's getting closer.