Which killer should I buy?

Clown for pop goes the weasel

Oni or blight for a fun and strong killer

Blight. He has Undying and he’s powerful and fun.

Do you have a playstyle that you prefer?

Do you like mobility and insta-downs but don't mind earning your (excellent) power in short bursts? Then go for Oni (plus he'll give you the Nemesis perk, which is both fun and useful for Obsession-related tome challenges).

Do you like hitting survivors with ranged attacks? Deathslinger is DbD's resident first person shooter. With the right perk setup, he's also a psuedo-stealth killer.

Do you like hitting survivors with LOTS of ranged attacks? It's pretty much all that Trickster does (though he's a bit difficult to use due to his unwieldy power and slow speed).

Do you like slowing/disorienting survivors with your fart-in-a-jar Tonic? Clown lets you pass gas all over the trial grounds. Plus he'll give you Pop Goes the Weasel, which is pretty great.

Do you like pinball? Blight is pinball. Plus Hex: Undying helps keep your hexes from, uh, dying.

If killer strength is a factor, then Blight, Oni, and Deathslinger are all generally considered to be very threatening when played well. Clown and Trickster can do fine too, but they aren't really on the same level (Trickster is actually thought of as being pretty weak right now).

Yesss

first and most important:

do you have all Perks or are you still missing the teachable perks of those characters?

if you are missing the Perks, i'd buy them in this order:

1. Clown
2. Blight
3. Oni
4. Trickster
5. Deathslinger

if you have all the Perks, i'd buy them in this order:

1. Oni
2. Blight
3. Deathslinger
4. Clown
5. Trickster