Hello,
it depends on what you're looking for, the latest Lamborghini Murcielago is without a doubt the BEST driving Lamborghini ever, but do you want that, an extremely fast car that isn't any more difficult to drive than say a fast BMW or a Porsche.
With the Diablo you needed some experience, especially the rear wheel drive versions were rather unforgiving when pushing to the limit, also the styling was different, more aggressive some people might say, the Murcielago is very smooth, almost too smooth to be a Lamborghini according to several people.
It's all just a matter of taste, and money, a Diablo will cost around 110,000 up to about $175,000 for a good one, while the Murcielago will still be near $300,000 for a few years to come.
My personal favorite is still the Miura SV, an awesome, completely timeless design in my opinion.
Kind regards