Biggest one is obviously Takano Hassei, who started as Fujimiya Hiroya / Ultraman Agul and a cameo in Seven X, and then Tezuka in Ryuki and Ichimonji in The First and The Next.
Kougami used to be Chief Tsutsumi in Gaia's XIG, while Gaoh was Commander Ishimuro (and Taiga Saejima for good measure)
In other relations, Nexus' Senjyu Ren is Punchhopper and the rabbit(?) orphenoc from Faiz, but everyone knows that.
Kobayashi Akiji, beloved Tachibana from original seven Riders, was also Captain Muramatsu in the original Ultraman. Kurobe Susumu, Hayata Shin / Ultraman himself, was a evil doctor in Black.
One that I didn't see coming was Moritsugu Kouji, Mr. Ultra Seven himself, or Tennoji from Blade. Talk about playing against the type.
And among some more obscure cameos/roles:
Kawaguchi Hideki, the kid from V3, also played the kid from Ultraman Jack.
Maria Theresa Gow, Georgie from Gaia, cameoed as the woman with the stopwatch from Foundation X
Shundo Mitsutoshi, Mizorogi from Nexus, was the good cop in W Returns Accel
Mitsushima Hikari, Wolf Imagin girl from Den-O, played robot girl Elly in Ultraman Max
Uchino Kenta, Teppei from Mebius, was Kintaros' original contract holder, remember?
Tanaka Minoru (R.I.P.) was the Commander Dopant also in Accel
Shin Ishikawa, Serizawa / Ultraman Hikari was also Kamen Rider Shin
Spoiler
Murakami Kohei has a cameo in Mebius as part of GUIS Sea team
Kagami Saki was Elea in Seven X and then Maya in Kiva.
Zolda also cameoed in Seven X
Karahashi Mitsuru, Kaido the Snake Orphenoc, was Grande in Ultra Galaxy NEO
Yamazaki Katsuyuki, Akai / Cactus Orphenoc from Faiz, was also Kazamori from Heisei Seven series.
I don't watch Sentai much, so can't really list some there, but I can bet that'd be a huge list.
I'm sure I missed a lot, too. Anyone know any other examples?