Grottomon/Gigasmon
Japanese: Tomohiro Nishimura
English Dub: Derek Stephen Prince
Derek Stephen Prince, or who I call Digimon's Man of a Thousand Voices. He's possibly voiced more characters in the series than any other. Other major characters he's done include PicoDevimon, Piemon, V-mon (and most of his evolutions), Impmon, and Beelzebumon.

Arbormon/Petaldramon
Japanese: Kenji Nomura
English Dub: Richard Cansino

Ranamon/Calmaramon
Japanese: Haruhi Terada
English Dub: Peggy O'Neil

Mercuremon/Sefirotmon
Japanese: Yasunori Matsumoto
English Dub: Doran Norris

Duskmon/Velgmon
Japanese: Kenichi Suzumura
English Dub: Crispin Freeman
Crispin Freeman gets high marks in my book of voice actors. He did an awesome job on Duskmon, along with all of his alter egos; Velgmon, Kouichi, Lowemon, and KaiserLeomon. In addition, he also voiced my favorite character from Slayers, Zelgadis, throughout most of its run (with the exception of episodes 1-10). .