As shown the secondary canon movie, Giratina has the advantage of physical strength but Dialga has something that Giratina can't break on his own: time manipulation. All Dialga needs to do is trap him in a time loop with no means to escape, attack repeatedly during the time loop so it happens again and again.
if talk about the pokemon from the filmes - Dialga will win because he seems to be mais powerful than giratina as he traps giratina in the time loop also giratina keeps on attacking dialga but he never attacked giratina (time loop for just not letting him out.Also dialga had a better moveset than giratina.Giratina,extremely powerful pokemon and the ruler of a world was getting defeated por megarig(machine used por zero),if shaymin did not help giratina was not able to defeat zero because his attacks (dragon force,dragon claw,will-o-wisp,aura sphere) was not so strong.if dialga was in place of giratina he would able to defeat zero on his own because his moves(roar of time,draco meteor,hyper beam,protect) were extremely strong also he would also traps the megarig into time loop.So I choose Dialga
Dialga of course, he is much stronger with its special typing that allows it to be resistant to both ice and dragon leaving his only weakness fire, he also have a better mover pool, roar of time can basically on hit KO giratina, it also have a high special attack to Mach up and even shattered palkia's energy pearl or something, causing him( palkia) to flee. Also in the battle against arceus, dialga actually saved everyone por sending ash back in time, he practically saved arceus single- handed so he can kick giratina's a$$ if he wants to.
In all honesty, Dialga was trrying its best to escape from Giratina so as to avoid a major conflict. Don't get me wrong, if Dialga wanted to really fight, it would damage Giratina badly, and vice versa. In an all-out brawl using all the strength that each of the two possess, the fight would end in favor of Dialga, as Roar of Time is virtually the most powerful attack in the Pokemon universe. Giratina would stomp a physical battle, but Dialga wins using the abilities it possesses and its typing allows it to be far mais resistant to damage than the ruler of the reverse world. This, of course is purely opinion, and I want everyone to know I'm not biased on either Pokemon's side. I respect their powers immensely, and Giratina has been shown to be able to manhandle Dialga, and likewise, Dialga can manhandle Giratina through evasion and using its abilities to inflict the mose damage possible.
My money is on Giratina simply because, in the Distortion World, there is neither o espaço nor time, and seeing as Giratina abides por these rules and can enter the Distortion World at any moment, and in the Distortion World there are no such things as "moments" I think even if Giratina were paused in this world he could still enter into the Distortion World, because if there is no such thing as time, then either Giratina exists at all in the Distortion World or he doesn't. Seeing as he created the Distortion World and you capture him there, he clearly exists in the DW. Because the DW has no time, he is in a state of constant existence there and can always come back even if killed in this world. That's my theory.
Dialga vs Palkia: it's a literal never ending debate. Dialga can reverse time, but what is a reversal of time but a recreation and rearrangement of space, over which Palkia has control?