KM is a bit worse (but not much). UK repair is the best, US a close second.

The repair "problem" for IJN and KM comes from a few sources:
1) Repair base ability: UK>US>IJN>KM (+11, +9, +8, +7). UK higher than US because
it has a chief repair which starts to accrue 1 extra sailor for the UK rep from level 65.
Also all UK sailors get +2 for repair thus you can't use the generic engy / BO amounts
below for UK.

2) UK bb don't carry AA. Most US bb don't carry AA. That means 2 bonus slots for
reps.

3) UK bb has one extra support slot to begin with, making UK the best for reps by
far. That is yet another reason why UK is best for beginners.

So I crunched the numbers a bit and here are the results (I created a spreadsheet
for these calculations).

Assumptions: all sailors are BVE with 109 vets
BO Base Rep stat = 9
Engies Base Rep stat = 10
Rep Base Rep stat = 11
No restorers
No Rep from gunners

Please check out the Adalbert's guide to see the graphs on repair ability. To sum up,
the repair rate of the ship is determined by the True Ability of every sailor on board
the ship. The 3 sailors with significant repair ability are the BO, the engies and of
course the reps. Restorers do as well, but this is IJN forum, hence discussion of
restorers not relevant.

The repair cap is at 35,000,000 true repair ability and is 280DP per second. This cap
is a dream for IJN (except for a level 120 Amagi crew with 2 engies and 6 elite,
boosted, 130vet reps). So since this is the IJN forum, lets not even refer to it as a
cap.

There is another important level of true repair ability level and that is 7,000,000. At
7mm the repair rate jumps from 38 dp per second to 78. With IJN, even at level 80
crew, the Yammy will be over 7mm. With Fuso (assuming that you are using one T-
slot for a repairer, 4engy 2rep at level 70 will put you just over this cap.

Over 7mm, the increase in repair rate for an additional unit of repair ability is linear.
The rate is roughly 8sd per second faster per each 1mm increase in true repair ability
(this could be wrong, I guessed at this by eyeballing the graph in the Adalbert
guide).

Here are some shortcuts you can use to calculate your total repair and make your
own engy vs rep comparison.

Engy (109 vets, boosted, 10 base repair)(works for US,IJN, overstated for KM due to
-4 repair at level 65 chf engy although chf engy gets 1 added sailor per level so that
compensates a tad, understated for UK because UK gets +2 rep for all sailors):
lvl50: 567,180
60: 704,686
70: 840,159
80: 988,233
90: 1,147,146
100: 1,317,739
110: 1,501,173
120: 1,695,206


BO (109 vets, boosted, 9 base repair, same levels as above)(same for all navies
except understated for UK which gets +2 rep for all sailors):
50: 643,215
865,985
1,117,875
1,398,885
1,709,015
2,047,230
2,415,530
2,812,950



Reps (109 vets, boosted, same levels as above)(IJN only)(I have calculated for other
navies but not published here, will share for credits ;) )
base rep 10
789,594
1,125,816
1,512,918
1,949,971
2,438,753
2,978,415
3,568,957
4,209,130


base rep 11
830,934
1,181,184
1,584,234
2,040,084
2,548,734
3,110,184
3,724,434
4,391,484


base rep 12
you are too lucky and I don't care about you :)

elites (base rep 13)
913,614
1,291,920
1,727,715
2,219,381
2,767,687
3,373,722
4,035,388
4,754,943


edits:
declaimed ability to use generic engy/BO for UK
adjusted generic engy for KM based upon Chf Engy getting -4 rep but +1 new sailors
per level
11/30/09: Fixed Ability Levels based upon correct boost formula from xHomicide