Ephesus or Priene (or both)

Raj, I've been to all four sites. Ephesus is the largest and most impressive, although I loved all of them. Doing both in one day depends on your stamina, but if you choose Ephesus, there are a couple of other sites in nearby Selcuk: there's a Byzantine castle, the tomb/basilica of John the Evangelist, and the ruins of the temple of Diana (okay, there's one big column).

From Izmir Ephesus is the nearer site. It would be very difficult to do everything in one day. Ephesus is huge in area and walking through it will take easily 2-3 hrs. Most impressive is its theatre where Paul preached and the Library of Celsus. Also worth visiting are the ruins of the Church of St John, and the nearby citadel and the chapel that is supposedly the final home of the Virgin Mary. It took us a full day to visit Prirene, Miletus and Didyma from Kusadasci. Three very different sites - Prirene is on a hillside set among pine trees and is definitely off the tourist routes (or it was 10 years ago when I was there). Miletus has an incredible theatre, a gymnasium and baths and some interesting later buildings (a caravanersii and mosque for example), Didyma is a beautiful and large Greek temple. If you are interested in the beginnings of Christianity then definitely Ephesus and area. If you are interested in a variety of sites from Greek through to early medieval, then choose Prirene etc.

Raj, Not having been to Priene, I can't recommend one site over the other. Having been to Ephesus, I can definitely recommend it. If you do go, three pieces of advice. One, there are tour guides outside the entrance for hire. The licensed guides have a photo ID hanging around their neck. It's well worth it to hire a guide, to explain what it is you're seeing. Two, the Terrace Houses, http://www.ephesus.us/ephesus/terracehouses.htm, are an additional entrance fee, almost as much as the regular fee. But they are well worth the extra fee. Three, bring a hat and some water. There's very little shade and no place to buy anything to drink once you're inside. Also, I would not recommend driving yourself. The roads aren't well marked and, while the Turkish people are warm and wonderful, they sometimes take the rules of the road as an advisory only.

I haven't been to Priene, but I have been to Ephesus and it's amazing. I would definitely say to go there. And as a previous poster suggested, do go and see the terraced houses.

I was in Ephesus in March and it was terrific! I concur what others have said a five star visit. also see the museum in town nr the bus stop. I believe you can take a public bus from Izmir but it may not be direct and slow.
you cannot go wrong w/ Ephesus it will be memorable allow 2-3 hrs.

I've been to both areas. If you only have one day and are traveling from Izmir then Ephesus is really your only option. It's a top notch site and would be my recommendation even if all of the sites were equidistant. In most of a day you'll be able to see the ruins, the museum in town and the temple of Artemis. The three other sites are to the south of Sulcuk and even Priene (the northernmost) will take a while to get to. None are really served by public transportation. Don't worry about renting a car. The roads to Selcuk are in good shape and there are good signs. All you'll need is a basic road map and maybe a quick primer on the toll road. There is regular public transportation between Izmir and Selcuk as well (buses). One note - Ephesus is about a mile or so outside of Selcuk, so you 'll have to walk or take a cab once you arrive. If you're staying overnight, some hotels offer drop-off service too. At the entrance you can hire a guide (negotiate on price) or just buy an English language guidebook. Have fun.

Raj, regarding public transportation, I took buses to all four sites. Local dolmus is particularly fun - tell the driver where you want to go and your fellow passengers will make sure you get there.

Thanks everyone! Based on your advice, I'll focus on Ephesus. I appreciate all of the information.