The consensus picks for most useful units in the new codex are:
Warboss with mega armor and Lucky Stikk
Painboy
MANz
Tankbustas
warbikers
Mek guns
Deffkoptas (normally lone)
FW units: Zhardsnark, gun wagons, kustom Stompa
Obviously, there are many votes for other units, but these are the ones that consistently get the best reviews and most love from the more esteemed ork players of Dakka.
Like most of us, you want to use the models you own, especially ones that are painted/converted well and the models you consider fluffy and fun. I'm not saying you should want to redo your list into a tournament contender or to throw out your beautifully painted nobz mob, for example. I just figured I would try to save you time researching these forums by summarizing the top pick units.
My only criticism of the list is there are too mant "extras" that are often not worth the points: red paint jobs, big bomms, and a big shoota on your slugga boyz nob? There's no option for that.
Also, for the Horde detachment, you need three Troops, I believe. You can take two combined arms detachments to get extra
HQ slots. (You would need to add two more troops.) Then you can split your Deffkoptas into singleton units and zoom them all over the board to grab Maelstrom objectives.