Meguma Ogata (X1)
ZERO
Wayne Dozier (X4)
It's actually Megum
i Ogata for X1's "Hadouken" voice. For some weird reason, though, that mention has recently been erased from Ogata's site.
As for X4's Zero, IIRC the credits read "Wayne Doster".
There was also some debate going on a while ago whether Shiraishi voiced either X or Iris in X4, but if Rock in MM8 is Shiraishi and seeing their voices are identical from one game to another, that's a moot point.
I know that for Rock himself, Shiina Hekiru (Hikaru in Magic Knight Rayearth) provided his voice in the "Wish Upon a Star" OVAs and "Power Battle" and "Power Fighters" as well as the drama tracks of legend with Mega Zaurus and all that. Reportedly people said Ai Orikasa did him in Rockman 8 and Super Adventure Rockman, Kaoru Fujino in both Marvel vs. Capcom and Yumiko Kobayashi for Rockman Rockman - as pointed above by Gonzo's post. Didn't read it until now. D'oh.
I also heard that they casted whoever did Axl in the CM~X8 dubs for Rock's part in Powered Up. Don't know the veracity of this claim though.
I had a listing of the voice actors from X4 and forward detailing each of their roles, since they always end up doing some of the bosses for good measure, but lost it because the DVD they were on farted kerosene (in other words, died all of a sudden), so the following list will be incomplete. Regardless, the main ones would be (japanese names first just for the heck of it, and if there isn't a parenthesis that character has not had a different voice actor for all of the games accounted here):
Alia - Rumiko Kasahara, Rumiko Varnes (X7), Mariette Sluyter (X8)
Axl - Takayama Minami (MUKURO in YuYu Hakusho), Lenne Hardt (X7), Jeffrey Watson (CM~X8)
Colonel - Jin Yamanoi, Matthew Meersbergen
Double - Either John O' Conner or Jeremy Felton for the US version
Douglas - Wataru Takagi
Dr. Light - Shozo Izuka (Rockman 8), Hirokazu Sasayama (X8), Tomohisa Asô (IHX), Randal Wiebbe (X8~MHX), Jim Byrnes (Ruby Spears)
Dr. Wily, Isoc - Takeshi Aono (X4 flashback, X6), Ian James Corlett (Captain N), Scott McNeil (Ruby Spears)
Dynamo - Showtaro Morikubo
Gate - Keiji Fujiwara
General - Ryuzaburo Otomo, Mark Hagan
High Max - Norio Wakamoto
Iris - Yuko Mizutani, Michelle Gazepis
Layer - Sonoko Kawata, Meredith Taylor-Parry
Lumine - Junko Noda, Elinor Holt
Palette - Haruna Mitsuma, Chris Simms
Red - Akio Ohtsuka, Barry Gjerde
Sigma - "Mugihito" (Makoto Terada / "Mugihito Tenchi"), Charlie Fontana (X4), Walter Roberts (X7), Dave Pettit (X8~MHX)
Signas - Hirotaka Suzuoki (X6, X7), Tsuneyoshi Iwatsuru (X8), Robert Belgrade (X7), Roger Rhodes (X8)
Vile - "Mugihito" (X8), Hiroshi Shimozaki (IHX), Roger Rhodes (X8~MHX)
That's keeping it 21XX. Next, 22XX.