live2travel:

No contest in my experience with all three..it's Kiev (Kyiv) by a knockout. It's striving hard to become a world class city (whatever that means), if they ever find a way to measure the attributes. Kiev has the most sights, the most history, pre and post SSR days. In two and a half days, you could run around day and night and not scratch its surface.
My most recent visit was the fall of '08...dramatic!

stu tower (a USA poster named "Orange" lives in Kiev..hope she chimes in here!)

I have not been to Lviv or Odessa, but based on my russian and ukranian friends' feedback, with Odessa you get the Black Sea and a really nice atmosphere.

Sights to see - I vote for Lviv.

Historically there are 2 European style cities in Ukraine: Lviv and Odessa...

I've been to both, and it's a tough choice, but Lviv made my revisit list and Odessa didn't. Don't miss the cemetery. See http://wilhelmswords.com/eur2006/index.html for my TR.

I have not been to Lvov, but I have been to Odessa 3 times ( in-laws live there). It's really not very exciting...different, but not exciting. Has a certain charm to it, but it's not Paris by a long shot.

Kiev would also be my primary suggestion but if you have to choose exactly between Lviv and Odessa and you travel in summer, pick up Lviv. Its architecture is charming and all attractions are in walking / short ride distance. Odessa's architecture is magnificent Russian-style, I don't like this. Additionally, Odessa is number two popular summer holidays destination in Ukraine (Crimea is number one), it's very crowded in summer and not completely safe. I don't mean that you'll be robbed right after leaving the airport or railway station but just keep an eye on your belongings.

Thank you all for your insight. We greatly appreciate it.
We are going to Moscow, St. Petersburg, Tallinn and Kviv. We needed some input on Odessa and Lviv. From the guide books I have read, I was leaning towards Lviv due to the amount of sights to see, but Odessa has the Black Sea.

Thank you all!!!!

Lviv !
http://www.youtube.com/watch?v=awctMMz61vY

Since you don't have a lot of time I would do Kiev. It's stunning. Architecture is divine and you can rent an apartment for relatively little in the center of town. Odessa is really something you should go to as a destination in itself. It's a resort city and you should take the time to enjoy it. If you need to figure out a place to stay, let me know. My mom and grandma just visited again in December and rented an apartment for fairly little money.
I am originally from the Black Sea area so if you had more than 2.5 days I would wholeheartedly recommend Odessa.

To see a TRUE Ukrainian city, you must go to Lviv. It's very different from both Kyiv and Odessa--the language spoke is Ukrainian It's beautiful with all the architecture and history. Some best European style cafes and bakeries are in Lviv, imo.

Just one remark: Lviv is a real mix of Ukrainian and Polish, so it's not a true Ukrainian city. But very delicate and charming.

Been to all three and I would do Kiev and Lviv for sure if times allows.

Lots to see in Kiev,it really is a nice place.

But I loved Lviv also.
I climbed to the top of the City Hall tower in Lviv,terrific views.

Like thursdaysd said ,"Don't miss the Cemetery in Lviv"

It depends of what do you want to see. lviv is old, ancient city with aged building, ukrainian traditions, narrow streets... Odessa is allways crowded, but has its own soul - its well-known center of humor, unusual dishes, jews, huge street markets

If we are talking about summer Odessa has more vegitation and outdoor discos (together with everything else). Talking about autumn and winter, Lviv is surely better because it has some (even outdoor) festivals, celebrations and so on.
So, as abovesaid, decide what you'd like to do and to see!