depends if the people you want/need to kill have shields.

If there is a 5/1/2 yasuo and a 2/1/2 karma support i would go serpents 2nd.

but if its a 1/0/0 mord top and a 0/0/2 sett support then i would probably go mythic 2nd as i can focus their mid/adc anyways, and then go serpents 3rd/4th item.

steraks doesnt usually require a serpents rush as it isnt built by champs until 2-4 items, a couple shieldbows i would for sure because its their first item, like yone mid with draven bot.

its very case to case, there is no golden rule. Look out for other champs on your team that can build serpents too, although it is usually the ad assassins job, especially a zed cause he can proc it easily before/during a fight