Who do you think could win this fight: Lucario vs. Darkrai

Hmm... Since most of Lucario's moves, which are fighting type, do not effect ghost types, I have to go with Darkrai on this one. However, if Lucario was at a higher level and/or knew not too many fighting-type moves, he could win.

Who do U think ill win: Latios v.s Rayauzard

Since you meant Rayquaza, I have to say him. He's the best there is to offer in the Hoenn Region, along with Deoxys. He can learn all sorts of moves, and he has high attack, Sp. attack, and a higher defense than Latios.

Okay, i would want to ask about the Gym battle between Roxanne and Ash in Rustboro City,Hoenn.
Though i have watched the whole battle more than once(or maybe 2-3 times or more) but still find confusing to understand how an electric type Pokemon Pikachu beat the Rock Type gym! Iron tail was definitely the twist there, but is Iron tail the only thing that affected on Nosepass? and how does Iron Tail works on a Rock/magnetic type Pokemon? And also, i saw that Iron Tail works on Rock Type but didn't actually got the magnetic part :/ (sorry if my question is silly :P)

I know it was wierd. What you have to understand is that the anime is way off sometimes from the game. For example, the whole Pikachu aiming for Rhydon's Horn in Cinnabar Island. In this case, Nosepass can be hit by electricity because he is only rock type. If he was part-rock, part-ground, then Pikachu's electrical attacks would do nothing to him. Is that a good answer to your question?

Nosepass is only a Rock type?! So, thats why Pikachu's attack totally affected.
Yeah, definitely your answer was good! Thanks :)

You are quite welcome!