If im not mistaken, the "best"/most appropriate atty to use at 5v is a "standard" atty.
the resistance on standard atty's range very slightly from what i have seen.. for example.. standard atty from
madvapes.com 3.2-3.4ohms
510 Spare Atomizer
but they all seem to be right around 3ish ohms...
now .. i do have some carto's that are @ 2.5ohms and they perform beautifully at 5v
i have seen/read recently people using HV (high voltage) atty's while
vaping at 6v.. from what i can see the HV atty's are around 4ohms... example... Nhaler.com @ 4.5v
High Voltage 510 Atomizer w/ 4.5 Resistance they also have an HV atty at 5.2ohms
I personally have 2 of the standard atty's from madvapes and they are both working strong aprox a month after purhase.
Good luck, hope this helps