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HGSS, BW2 or LA (hopefully if proven by ZA, any Legends game).
Thr former two, especially BW2, are some of the only ones that don't feel like a slog to start up beyond the first 15 minutes, even if said sequel is bogged down by the infamous Pokéstar Studios tutorial after the 2nd gym. best sidemode excluding the atrocious tutorial tho, and in general the 'pointless' side content of HGSS and BW2 like said movie puzzles, the musicals, Pokéathlon, bug catching contest, Join Avenue, the best Safari Zone, or even the funfest missions are the best parts of these games besides the mons themselves with how boring as sin conventional wild encounters and trainer battles are (hence why people love safaris and battle facilities in these games).
Meanwhile, in spite of looking like dogshit and being quite empty in most places (especially the first fucking area of the game), Arceus feels the most 'alive' of any game to date, especially among the Switch or in general modern 3D games. The small details and easily missable side content is why I'm a fan of Iwao's games, even with USUM's many other flaws, and even with the slow first half to full hour until you catch the Sinnoh's early trio, I could similarly play it constantly from game feel alone, which is why (ignoring the comical performance) SV forcing battles even with back strikes makes me so upset more than its other gameplay faults.