If youre thinking for current/ future raids then i would suggest Groudon and Articuno. Groudon with solarbeam will be a good choice against Kyogre and Articuno will be great for the future Rayquaza,Latios, and Latias(who knows when they will be released though.)
Entei is overall an amazing fire attacker, but fire types arent in high demand currently. He would be good for Registeel and Regice however.
Ho-oh is kinda niche but not toally worthless. Its still a very good "grass type" option if it knows solarbeam, again somewhat useful for Kyorge but not the best choice.
I personally already have Articouno and Entei powered up over 30 from their releases and ill also power up Groudon. No plans to power up a Ho-oh anytime soon though.
Right now I'd go with Groudon since it's one of the best Kyogre counters with Solar Beam. Articuno is going to be good later, but I would not power it up until Rayquaza raids pop up. Same can be said for for Entei and Registeel/Regice raids. Ho-Oh is niche, it does sport very good TDO in many raids but if you don't feel like you need a tank, don't power it up.
I'd consider Groudon and Entei worth maxing eventually. Articuno is going to get severely overshadowed by Piloswine in gen 4 and Ho-Oh doesn't seem like it'll ever be supremely useful if it doesn't get a better moveset.