the easiest answer for me is the vee-twin moto morini 3 1/2, because i have one and have always loved them dearly - well, the early ones at any rate. aim for spoked wheel models - about 1973-76 - but the most desirable are the early Sport models with drum front hubs (the Sport has double-sided single leading shoes, the Strada [tourer] has a twin-leader)
however if you really want a 250cc you cant go past a Ducati desmo. the pre-desmo single-cylinder dukes are a painful bike if you're anything over 5 foot tall, but the mkIII'D' is a dream.
both the morini and ducati are available in other capacities between 125 and 500cc, but the 350 morini and 250 duke are the most sought-after in their respective ranges
also bear in mind that whilst the italian bikes of this era are very reliable, they are an enthusiasts machine - be prepared to take good care of them, checking oil and tappets regularly, as well as the electrics ... but then, if you own italian cars im sure you can guess what youre in for! good luck.