Gulbis --> His serve flows smoothly, as soon as he can work on it more, mainly by adding a bit of more variation it will be a huge help for him. He´s still unmature but has a lot of potential and flair to me. Excellent prospect, still soon to know if he will be capable of being a regular top 10 player, I´d say he should be.
Korolev --> A real gem, you´ll hardly see such a young guy hit that hard off both wings, he needs to work on that serve of his, he has power on it but it´s too predictable imo, he has to improve his defensive skills, add a bit more of variety, (mainly just to avoid killing the ball when it´s just impossible) , but I´m convinced this kid has it all to be a solid top 10 player.
Cilic ---> Many people say this kid is crap blah blah blah. He´s actually the Croat I like most, above Ancic, and I think he will have a very good career as well. Top 20 material.
Young ---> Talent was always there and he´s starting to fulfill his potential, he will be very good on hard courts, don´t know how he will adapt to slower conditions though, remains to be seen.
Tsonga ---> Very dangerous guy to play against on fast surfaces when he´s on. I don´t think he can succeed on clay and looks like he´s injury prone, that will hamper his ranking. Maybe top 20 at some point of his career, not a regular though imo
Querrey ---> Same as Young, less talent than Donald, but with that serve and those good groundies for a guy of his height, I have no doubt he will do fine on hard courts. Top 20 at some stage in his career, probably not a regular though as well as Tsonga
Zverev ---> Good serve and a lovely touch at the net, a bit rusty at the baseline, but I feel he has a good margin to improve there, once he serves more consistently in a couple of years´ time and improves his baseline strokes (I´m sure he will) he will be a very tough guy to play against on fast venues. Problem is his style won´t work on clay, but I´d say at his peak, easily a top 15-20 player on fast surfaces.
Bolelli ---> This guy has loads of talent to me, he can play both on clay and on fast surfaces (I think his game is better suited to fast venues), he has a good first serve, his fh is a joy to watch, he´s got flair and he´s improving his backhand lately. Apart from a bit of more experience, he needs to improve his footwork and his defensive skills to me
Very difficult to answer the question in this thread, I´m stubborn and I´d say Korolev will have the more successful career, and if somehow he fails at my expectations towards him, then I´d say Gulbis although I´d happilly take if Cilic turns out to be the best Croat since Ivanisevic