Everybody loves Dialga for some reason. Giratina too. :/ I loved Palkia. I like Giratina too, but Dialga was just...ugh. XD;
Firstly, Palkia is part water-type. I LOVE water-type Pokemon since they're so powerful and have a great moveset. I also like Spatial Rend, since it's a pretty original move, unlike Dialga's Hyper Beam copycat Roar of Time. It also looks amazing. It stands upright on two-legs, giving it this dignified look. Dialga looks like a disheveled stray dog (no offence Dialga, but the truth hurts :[). And Palkia had its strengths in the battle. From the top of my head, I can think of only one weakness Dialga has (Dragon), compared with Dialga's 2 (Ground and Fighting) and Giratina's 4 (Ice, Dragon, Dark and Ghost).
Giratina was pretty good for battles. It has a lot of weaknesses, but it makes up for them because of its bulk. It's probably better than Palkia in battles, probably because of my style of battling (sit it out). Then again, the 4 (I think it's 5) weaknesses of Giratina make it pretty vulnerable to a powerful attack by the foe.
Overall, I guess I prefer Palkia over Giratina, and Giratina over Dialga. I guess it's because I've played Pearl and Platinum a number of times compared with playing Diamond just once (and subsequently having more experience with Giratina and Palkia over Dialga).