I've been using defensive Ho-Oh a bit recently and it's really impressed me a lot more than I initially thought it would. I've almost exclusively ran offensive variants of Ho-Oh since Gen 4, and Fire/Flying is traditionally seen as poor defensive typing, but Ho-Oh has a lot of things going for it that can let it act as a pretty fantastic wall that can deal with some of the most dangerous sweepers in the game.
Ho-Oh @ Heavy-Duty Boots
Impish - Regenerator
248 HP / 208 Defense / 56 Sp. Def
-Sacred Fire
-Whirlwind
-Toxic
-Recover
Ho-Oh has the ability to check a ton of the most dangerous threats in inclusive formats, like Xerneas, Calyrex-S, Marshadow, Eternatus, and Necrozma-DM. This ability to blanket check a wide variety of sweepers combined with Regenerator lets Ho-Oh act as an amazing defensive pivot for many team archetypes. This is the particular set I use and it's pretty straight-forward, but there are other options to consider as well.
Sacred Fire is generally your best STAB; Brave Bird is a little stronger and has perfect accuracy, but chips into your HP, undermining your staying power against powerful offensive Pokemon, and lacks Sacred Fire's 50% Burn chance, which is incredibly useful for punishing switch-ins like Tyranitar, Zekrom, and Zygarde-C. Carrying both moves is an option, though.
Whirlwind is key to shutting down set-up sweepers like Xerneas, Bulk Up Marshadow, Cosmic Power or Meteor Beam Eternatus (once it's used its Power Herb), and Dragon Dance Zygarde-C. Unlike other phasers, Regenerator makes it very easy for Ho-Oh to pivot into these threats, phase them away, and maintain enough HP to do it again and again throughout the match without getting worn down.
Toxic is used to wear down bulkier switch-ins like Kyogre and Yveltal who aren't as crippled by a Sacred Fire burn. Thunder Wave is a more crippling to a few key threats, such as Eternatus, as well as preventing Ingrain or last-mon Xerneas from pulling off a sweep, which Whirlwind cannot do. Defog is another option that allows Ho-Oh to remove entry hazards. Personally I carry it on another Pokemon, so I don't usually feel the need to carry it on Ho-Oh, but it is absolutely an option if you're looking for a Defogger.
It seems to be pretty popular to forgo Recover in favor of one of these other moves in the current metagame. While I can see the reasoning behind this, it does force you to pivot with other Pokemon a bit more aggressively than you otherwise might, and Recover makes sure that Ho-Oh stays healthy even in the face of multiple set-up sweepers that it needs to check throughout a game, so I usually do carry it.
The given EV spread survives a +2 Thunder from Modest Xerneas, with the rest being dumped into Defense to bolster your ability to check physical threats like Marshadow and Zygarde-C. Moving 16 EVs from Defense to Speed allows you to outpace a paralyzed Calyrex-S, which can be useful if you run Thunder Wave.
Ho-Oh is a very easy Pokemon to fit on many team archetypes. Redundant checks to certain threats, especially Calyrex-S and Xerneas, are definitely immensely appreciated, as with a little bit of luck and/or some chip, these threats can overwhelm Ho-Oh, but that's basic team-building. Playing Ho-Oh optimally generally involves a lot of pivoting to scout for moves like Rock Tomb on Marshadow or Toxic on various defensive Pokemon like Necrozma, so other defensive pivots like Tangrowth, Toxapex, Blissey, and defensive Yveltal make for fantastic teammates. If you choose not to carry Defog on Ho-Oh, you'll probably want it somewhere else on your team; while Heavy-Duty Boots is fantastic for Ho-Oh, having them knocked off could be devastating without a way to remove hazards.
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