You don't say what specifically interests you, but here goes according to my tastes:
In general terms, Belarus does not have many interesting places. I would not miss Minsk, it's a nicely preserved Soviet city. Hrodna has a few nice spots. I found Brest pretty dull, except of course Brest Fortress which is definitely worth visiting. There is some nice forest between these two towns where you have a chance to see wild wisent (European bison).
For nice roads, the whole south is rather beautiful, I especially liked the drive from Pinsk to Mazyr; again we're not talking about anything spectacular.
You can see my account of Belarus (I went all around the country) here:
https://eurasiaoverland.com/2011/03/19/26/
Ukraine has slightly more to offer; again, I like Kyiv but from a sight-seeing point of view Lviv is more beautiful; one of the nicest-looking cities in the Former USSR in my opinion.
For scenery, the Carpathian Mountains are very beautiful; almost certainly the most beautiful part of the country now that the Crimea is off limits (without a detour round through Russia).
I thought Chernivtsi was a very beautiful city and would put that on my route. You can then head past the castles in Khotyn and Kamianets Podilskyi.
Odessa is a great place, very atmospheric and distinctive. A great mix of seedy port atmosphere and elegant buildings.
Accounts of Ukraine are on my website here:
https://eurasiaoverland.com/category/ukraine/
Note that Ukraine has some truly awful roads; some are so badly potholed that they would not be much fun to ride on with a heavy bike. They also seem to have deteriorated (I visited this year and found them far worse then in 2011 when I did most of my travel in Ukraine). The areas around Moldova are some of the poorest in Ukraine, so expect the roads to be awful.
Moldova is a country I really enjoyed (I drove through much of Transnistria, which was beautiful and interesting). There's a nice castle in Soroca, and the cave monasteries of Orheil Veichi and Tipova. I liked Chisinau and Gagauzia too. My favourite memory though is of being invited into peoples' houses to drink home-made wine.
Moldova journey here:
https://eurasiaoverland.com/2011/10/31/31/
Romania is not a country I have anything good to say about, so I will not say anything.
Good luck
EO