How fast people forget that Tszyu dominated for 8 years and had all three belts, Forest did not. Forest had Sugar Shane's number but also lost to Mayorga twice in a row in his prime. Tszyu record and reign are much more impressive.
Tszyu wasn't as wild as Mayorga, he'd have to attack Forrest like Mayorga did in order to get him like that, or else he gets outboxed or even knocked out. BTW Mayorga is not that bad, he made Mosley and Cotto look pretty old and worn. He helped a lot of fans think Mosley was too old for Margarito; and I thought Cotto took too long to get an obviously shot and old Mayorga out of there. A lot is made out of him being knocked out by Trinidad and De La Hoya, the thing is Trinidad and De La Hoya knocked out almost every one, barring their losses.
forrest fought at welter, kostya did not. Also kostya was older than forrest and with a more extensive amateur background than forrest.
Amateurs don't equate to the pros. Plus that amateur fight wasn't a KO or anything of the sort, but a somewhat exaggerated victory. In the pro ranks, Tszyu got beat down by a guy that Forrest easily beat. I think Tszyu would have been too small for Forrest in the Pros, and I think Forrest would have won a comfortable decision, maybe late stoppage.
I never brought up their amateur careers. Tszyu did get caught and lost to Phillips, but I wouldn't say that means anything because Forest beat Phillips in a 12 round decision. Sharmba Mitchell beat Phillips, and Kostya murdered Sharmba twice, with 7 and 3rd round knockouts. Tszyu reigned supreme for 8 years after the loss, and did not lose for 8 years. Tszyu was a very good boxer, and tricky counter puncher, which is how Judah got knocked out in 2. I think the other people posting are making too much of one weight class considering that Tszyu was a big puncher and Forest was not