His best year is arguable. A lot of people say it's 98, when he won his first Olympia title, but he was a lot more bigger and rounder in the 99 Olympia and GP shows (just not as dry and conditioned as in 98). He looked insanely big in 2000 and even 2001, but his conditioning wasn't at it's best in those contests. Then you have his 2003 Olympia and 2001 AC shape, each of which had their own pluses and minuses.
His arms started getting "smaller" only after 2004. I personally think his 99 and 2000 Mr Olympia look were the sickest, since in 98 he was not as muscular.