Ho-Oh as Regi-trio counter

There's tons of top counters to Regice.

Sunny: Moltres, Entei, Flareon.
Partly Cloudy: Smack Down Ttar (watch out for Focus Blast)
Cloudy: Machamp, Breloom, Blaziken (Fighting moves), Hariyama

If you really like Ho-Oh, power one up, but the Fire Blast moveset is weak. There's lots of other options for Regice, as listed above.

Ho-Oh is not really worth mentioning as a fire pokemon. Against Regis you're better off just powering up your Machamp squad more, it's the best nonlegendary counter in neutral weather.