Spain vs Italy vs GreeceWatch
Remember though, if you're going to Greece, take several hundred euros with you...
Personal choice though. =l
I'm planning to go on a holiday with my friends to either of these countries but I don't know which one to choose.
Greece is definitely the cheapest option, but if you want to go to more "party resort" style you should go to Spain.
Italy is more cultural, more expensive, better quality hotel. There is partying, but there is more of that in Spain.
If you want to go to cities, go to Spain or Italy. If you want more remote and nature, go to Greece or Italy.
Italy is the most expensive of the three and carries a reputation of being a 'refined' holiday destination. However the food and scenery (especially in the south) are amazing. Spain is probably the best set up for local bar culture - whilst in all three locals eat out very late, in Spain bars are often teeming at midnight with people of all ages even on work nights. In Italy and especially Greece sitting and having a drink is more of an old man thing to do.
Agreed with the above that Spain is the best of the three overall for cities. As Spanish culture is quite heterogeneous among the different regions (much, much more than in France, Italy and Greece), all major cities have a unique charm about them. Try telling someone using just photos that Santiago and Seville are in the same country! All three are decent for nature, with Greece's probably being the most accessible to the tourist without a car. If I had to pick one natural sight to see in any of the three it'd have to be the Santorini caldera.
Overall I'd say the following. If you're on minimum budget but maximum fun, Spain. If you want decent lodgings and upscale food, Italy. If you're on a slightly smaller budget but want to walk in on amazing natural sights like the islands, Greece.