My opinions- with short reasonings
Assumptions- playing 2k+ points, any time I say freebooterz I am assuming you are playing mostly freebooterz as it becomes harder to get their klan effect the less units of them you have your army relative to other units.
A) Boyz Trukk boyz (10 models) If shooty deffskullz/freebooterz [badmoonz re-roll ones is not as good for 10 models with a big shoota/rokkit] If choppy Goff, Evil sunz. For 30boyz; if choppy Goff/Blood Axe For shooty Bad Moonz/Freebooterz
B) Nobz Shooty nobz- Bad moonz/Freebooterz, choppy nobz- Goff/evil sunz. Honorable mentions- Skorcha blood axe nobz (fallback and shoot is pretty good with flamers) 5 model nobz with ammo runt and 2-3 rokkits as deffskullz
C) Gretchin Freebooterz are the only ones that can benefit klan traits in any way, but you need them as bullet shields for any foot slogging infantry of same klan
D) Mek Deffskullz- re-roll to hit
E) Burna Boyz Evil Sunz if you tellyport them- the +1 to charge, Blood axe or goff if you put them in trukks. If a trukk falls back the burna boyz can still shoot burnaz while embarked if they are blood axe, and getting extra burna attacks with 6s from goff is pretty good.
F) Tankbustas Freebooterz, Blood Axe (10 tannkbustaz is PL1 so you can ambush them for 1
cp), if you run
MSU of 5 units Deffkull or freebooterz again
G) Meganobz Evil sunz, Freebooterz, Goff. Getting more attacks on 6s with powerklaws is always good, Evil Sunz for added movement, especially charge after da jump or tellyport. Freebooterz because if you get +1 to hit in assault phase it offsets the -1 from powerklaw.
H) Kommandos Evil sunz, blood axe seems like it would be good because you are in cover past 18" from shooting, but you often will be shot by what you are ambushing in near which will be within 18"
I) Warbikers Evil sunz, Freebooterz, Blood Axe. Bonus movement from evil sunz is huge for this unit, going to
BS 4+ with that many shots as freebooterz is insanely good, and being blood axe to fall back and shoot is really good- also charging something, then falling back 14" to shoot the next turn is a lot of moevement across the table as blood axe
J) Kustom Boosta-Blastas Blood axe, Freebooter. You want to charge and use the flamers, getting close to flamers means you can be charged before you charge so being able to fall back and shoot or charge is strong. Freebooterz because this is a unit that has a high chance to kill a screening unit and grant the freebooterz buff within 24"
K) Shokkjump Dragstas Evil sunz or Deff skullz, the movement you get is crazy and tellyporting on a 3+ is good utility, the rerolls for the low shot high strength attacks with deffskullz is strong
L) BoomDakka Snazzwagons Freebooterz, same reason as Kustom boosta blasta, Bad Moonz due to number of shots
M) Megatrakk scrapjets Deffskullz, blood axe, freebooterz. Being able to fallback to charge with drill or shoot rokkits is utility, this is another unit which is good at killing 5 model
MEQ for freebooter buff within 24" in assault or shooting. Deffskullz really benefits a lot with low shots
N) Rukkatrukk Squigbuggies Freebooterz, Bad Moonz. This unit is kind of a mediocre unit that can do pretty mediocre at various targets.
O) Stormboyz Evil sunz, Goffs, Freebooterz, Blood axe. Evil sunz for the +1 to charge when they deep strike in, goffs for more attacks on 6s and Zag grants morale immunity. Freebooterz because they pair well with letting you have a shot at killing some unit across the table near freebooter other units to give the 24" +1 to hit buff. Stormboyz make good use of the blood axe fallback or shoot, since they have fly they can do both but falling back with an unit which can ignore models and has large movement is really tactically powerful.
P) Deffkoptas Deffskullz if they have KMB, or Rokkits, Badmoonz for twin big shoota. Honorable mention evil sunz/blood axe for utility (evil sunz +1 to charge when they arrive, blood axe because with fly you can fallback and get to shoot AND charge if you want)
Q) Mek Gunz Freebooterz, because they can give +1 to hit buff if they kill something
R) Battlewagon Too situational/loadout dependent to pick.
S) Gunwagon Freebooter/Bad Moonz/Blood axe Assuming you took a big gun and 4 other gunz getting +1 to hit as freebooter is big, badmoonz re-roll ones benefits many shots, and blood axe lets you fallback and shoot if something assaults you
T) Bonebreaka Freebooter, Goff, Blood axe Evil sunz +1 to charge from evil sunz is good but ramming speed is better, extra attacks on 6s is pretty big from goff with the amount of attacks you can get with a bone breaker. Blood axe being able to fallback and charge is pretty huge unless you get wrapped. Freebooter, because this thing is good at granting +1 to hit in assault phase to units within 24"
U) Trukk Evil sunz for movement, trukks arent good at shooting or assault- but any klan that you might need to transport would need you to run trukks from that klan except freebooter which can ride in ork transports freely
V) Killa Klans Currently freebooter, because they can grant the +1 to hit buff if they kill something
W) Deff Dreads Shooting loadout is deffkullz or freebooter, you could do badmoonz with 4x big shootas or rokkits but you are either wasting points on something that is not worth taking or deffkullz/freebooterz is doing it better. 4x close combat weapon is evil sunz, goff, or bloodaxe Goff and evil sunz being the better pick. An unit of 3 deff dreadz is 2
cp total to tellyport, and gets three chances to pull off a charge when it comes in within 9", +1 to those is really good chance to get 1-2 in.
X) Morkanaut Deffskullz, or freebooterz. They have the best bonuses for low rate of fire high damage ranged attacks.
Y) Gorkanaut Bad Moonz, freebooterz, or goff. Bad moonz is a better high rate of fire shooting bonus, freebooterz because +1 to hit in either shooting or assault is good. Goffs for the bonus hits on 6s in assault when you have 18 attacks is good.
Z) Dakkajet Bad Moonz or freebooter. Honorable mention for blood axe (+1 armor save beyond 18") deffskullz (Reroll, but 6++)
AA) Burna Bommer same as dakkajet
BB) Blitza Bommer same as dakkajet
CC) Wazbom Blastakjet Deffskullz or freebooter
DD) Stompa This unit is so bad for its points it just doesnt matter. I would argue blood axe is the worst choice here as you can already fall back and shoot/charge, and the +1 armor isnt going to help as much as getting an invulnerable save or having a 6+++
EE) Lootaz [you left these guys out

] bad moonz for re-roll and strat, freebooterz if you want to have lootas and save on
CP.
Thoughts-
I didn't really mention snakebitez, but I feel snakebitez is interchangeable with freebooterz at 2k. The +1 to wound stratagem in any phase is similiar to getting +1 to hit at the end of all the mathhammer, and the 6+++ is a good all around buff for any unit. The 6+++ is a big deal in tournaments. A lot of players have an idea of what average rolls for their units do against
GEQ,
MEQ,
TEQ, Knight-EQ based on pre thought out saves. When you start reducing their average effect by rolling a few 6s here and there it messes up their plans, it's not huge but things like this are under-rated factors people ignore for "re-roll 1s" etc.
I think its better to generalize what klanz are good at things than what are the best klan selections.
Best All rounder/hybrid shooty/assault - Snakebitez/Freebooterz honorable mention Deff Skullz for low dice count high damage attacks and because their elite/heavy/fast infantry are ob sec. Either klan trait or stratagem can allow you to benefit in either Shooting or Assault when you with the right situation. Monster hunterz is any model with 10+ wounds, which tend to be the models which gives ork the hardest time.
Best Shooty- Badmoonz for high rate of fire units (lots of dice of same attack) Deffskullz for low rate of fire high damage attacks. i.e. mobs of 30 shoota boyz with 3 big shootaz bad moonz is good, a mob of 10 trukk boyz with a rokkit and a nob that has a kombi rokkit deffskullz is good. Honorable mention, Goff Skarboyz boyz with Shootas. Yes Shootas. They end up causing as many wounds as non skarboy choppa boyz but have the shooting output of shootaboyz to back it up against T4 and T5-9. 30 Skarboyz vs T4 = 90 Attacks, 60 hits, 40 wounds. 30 Non skarboyz with choppas vs T4 = 120 attacks, 80 hits, 40 wounds.
Best Asssault- Goff. Honorable mention for Evil sunz due to +1 to charge from reserves, and getting across the table fast.
Freebooterz caveat- you need to run all freebooterz to really benefit from their buffs. It is possible to benefit with fewer, but its harder to make sure one freebooter unit kills something to give the buff to another 1-2 units, in effect you will often not benefit from the klan trait at all.
FWIW most of the units I play are Freebooter, blood axe, or Evil sunz - not all in the same list.