I really like Marshadow after using it a lot in Pokemon Reborn. Its signature move Spectral Thief is so good to if you have an opponent that is using stat raising moves. So not only are you hitting with a 90 base power move but you are also taking their stat buffs first so if they used sword dance well congrats your 90 base damage is now 180 before even hitting.

It already has good attack and speed stats too at 125. So you don't even need the stat buffs. They just make a good pokemon even better.

As a kid, Ruby was my first game, so naturally Groudon was my favorite legendary Pokemon. I still like Groudon a lot, but I think my favorite has to be one of the early mythicals. Honestly, I think I might pick Jirachi.

I was a big MewTwo fan growing up and all through HeartGold/Soulsilver it was my favorite Pokemon, let alone legendary.

Then the Shaymin movie came out, and I haven't been the same since. I love Shaymin, both in lore, design, and moveset. It really is the perfect Pokemon to me, and has been my favorite ever since.

Double post I am ashamed.

To not be completely spammy, I also like Lugia thanks to Pokemon the Movie 2000! That was my favorite Pokemon movie when I was a kid.

My favorite of all time has to be Suicune. It was the very first one I ever encountered in the games and I love its design too.

Out of all the legendaries, i really like Kyurem. The Gen 5 games are my favorite to play through. I always went to capture him throughout most of them. I love his design and his Black Kyurem design as well.

My favorite legendary pokemon used to be Raikou but after using Tornadus in its Therian form (bird) it quickly shot up to be my favorite. I have won many difficult gym battle with that boomer bird on my side.

A lot of good options here but I'd probably go with Dialga. Pokemon Diamond on release was the second Pokemon game I played, after Yellow, and I don't think I ever caught Mewtwo in that game... although I think I did get Articuno, but I was so young that I could hardly tell the difference lol. Using Dialga for the first time, and especially Roar of Time, almost felt like I was unlocking a cheat code in a game that I've played for so long because it was just so much more powerful than anything else little kid me had experienced then. I'm pretty sure I got through the rest of the game just shoehorning it through every single remaining fight lol. For a little while I was a dirty little hacker who would hack in other legendaries and then challenge people to fights in online forums and chatrooms (definitely pre-Vizzed. I'm recalling a forum I used to go to before this one)... and promptly get my butt handed to me lol. Dialga was always on the squad without fail though, I do remember that.

It does also help that time-related powers are easily my favorite flavor of super power in broader fiction, so it's even neater to me that he's the god of time.