UngeheuerLich
Legend
Super high initiative is always fun!
Gunslinger Proficient in Initiative Rolls
Chronomancy +Int
Gloomstalker +Wis
Swashbuckler +Cha
So with 20s in each stat, plus Alert AND ioun stone of mastery and stone of good luck, on top of the Alert feat, you're talking an initiative bonus of +33. (Prof 7, Luckstone 1, Dex 5, Int 5, Wis 5, Cha 5, Alert 5)
If you can't, for some STRANGE reason, score 20 in 4 stats that doesn't even cover Constitution then consider doing it without attributes.
Rogue 11 Gunslinger 7 means you can reliable talent the roll and, with the alert feat, have a +11 to initiative guaranteed to roll 21. (of course if you dex 20 cha 20 and luckstone this becomes 32, mastery iounstone of 33)
Also Bard, jack of all trades or champion fighter. I think all abilities are low enough to fit at least the bard in.
You should also be able to get advantage on the check by some means.