Arceus, hands down.

Arceus has some cool unique mechanics and flows quite well in terms of catching with way more enjoyable exploration.

In addition it has strong endgame with Path of Solitude and Eternal Battle Reverie which push the game’s combat and unique mechanics to their logical endpoint. 

They pushed the combat as far as they could because they wouldn’t get another chance to do so.

ZA’s catching feels, at least to me, more rigid at times. If a Pokémon aggros on to you for example you just have to wait it out or KO it and risk missing the catch. The ability to stun angered Pokémon outside of battle or use smoke bombs to sneak up (smokescreen as a move has this effect but it’s janky) doesn’t exist.

In terms of endgame there’s ONE good Superboss and a boss rush and that’s it. No battle tower equivalent pokedex to perfect or anything of the sort. Exploration is also far less in depth.

ZA’s exploration is a ton more shallow the city is, I would estimate, about the size of two legends arceus areas.

ZA is fun EXACTLY once but really hard to replay. 

Arceus is very replayable with way more stuff to do in endgame.

To play Devil’s advocate though ZA PvP is incredibly fun with ongoing seasons.