For me, Rattata has always been one of the most annoying words to say. I think my friends and I all say it differently.
Here's how I say it: "Rat-at-ah." Maybe that's one of the reasons I've never cared for the guy...
And then Bidoof, which I say as "Bid-oof," though I know people who say it as "Bee-doof" or "Bye-doof." Bibarel has a similar issue.
Then Regigigas, which I say as "Reh-jee-gig-ahs," but I know someone who says it's "Reh-jee-guy-gahs" or "Reh-jee-gig-ee-ahs" and sometimes he just adds "gig" sounds in there for no reason. Same with adding "at" sounds in "Rattata."
When I was a kid I would always say "Feraligatr" as "Fur-ali-gator," but then I learned that there was a word called "feral" that described things that were viscous/wild, and decided that that was how you say the first part of his name.
And I dunno about this one, but is Victreebel's name supposed to be said like "Victory-bell?" I just say it like "Vick-tree-bell."
Seriously, Arkeeus? IT'S NAME. IS DERIVED. FROM ZEUS.
And I don't know about you, but I don't pronounce the father of gods and men as Zay-us.
Rattata - I say: Rat-a-tata xD (been saying it since '98 )
Regice - I say: Regi-ice
Arceus - I say: Ar-see-us
Rayquaza - I say: Ray-kway-zuh
Raikou - I say: Rye-koo
Zoroark - I say: Zo-roar-k (just recently got in the habit of saying Zoro-ark xD )
Missingno. - I say: Mis-sing-no (strange I know, but I was 6 when it was discovered xD )
Hmm... I don't often pay attention to official ways, as I never really watch the movies or the anime. I simply pronounce the name in the most logical (British English?) way, and if I'm unsure, I listen to how others here say it or derive it from how we pronounce related words. To me and a few of my friends, the English accent really makes some of the names sound odd if pronounced in even the "legit" ways, so frankly, if I feel I'm doing the Pokemon more justice, I will pronounce it in a way that sounds less stupid.
Giratina = Jeer-a-tina
Raikou = Rai-ko
Suicune = Soo-wee-coon
Rattata = Ra-tat-ah
Reshiram = Resh-i-ram (with the i like that in "it")
Gardevoir = Guard-eh-vwahh
Altaria = Al-teh-ree-a
Milotic = My-loh-tic
Illumise = I-lloo-mise
Kyogre = Kai-yoh-guh
Arceus = Ar-kay-us (Why? "ass" here is "arse", pronounced in the same way a lot of Americans say Are-see-us, which is just awkward and insulting to Arceus if you ask me because with a common British accent it sounds like "arse-say-us" or "arse-eis". Not cool.)