You have some great options.
You can go with Ho-oh itself, Ampharos, heracross or Lapras.
Let's go over all of them.
Ho-oh: it's the box art legendary. Having great stats, it can help a lot against will's jynx and few other elite four members.
Do note it's almost useless against champion lance as all his Pokémon don't mind fire much. You can fish and get burn to cripple his Pokémon though.
[email protected] charcoal or nothing
- sunny day
- brave bird (if you have heart scale, do get it.)
-pretty much anything like extrasensory or filler
Ampharos: it's really good and found after 1st gym. It helps against most trainers in the region and also helps catching ho-oh ( with thunder wave)
[email protected] magnet or nothing
-Discharge or thunder (depends what you like, more power or better accuracy and para chance)
- Signal beam
-filler, some hm like strength probably
Heracross: pretty much pain in the neck to get. Best place is the town with 2nd gym. See youtube for less pain. But it's completely worth it. It's really good in this game. Helps against dumb milktank of Whitney, jasmine, pyrce and dark type elite four.
[email protected] anything useful
-brick break until close combat (better to have both in elite four battles)
-mega horn (you can keep close combat here in elite four time)
Lapras: mainly for 3 dragonites of lance. They are pain to play against and it's very satisfying to one shot them with ice beam LOL
[email protected] mystic water or never melt ice
- confuse ray
-filler, something like whirlpool or body slam
You can choose any.
Hope I helped!