Interesting. Have to make a couple of assumptions:
1. Don't have to support RN (no carrier requirement) - if we do add F35C to the list.
2. Wild ass guess at what things cost.
3. Ignore the "keep in NATO!"
4. Has to be available nowish - no dream aircraft
5. We aren't going to war with anyone too capable like Russia!
Fast Jet:
1 multi role type jack of all trades - F15E or Typhoon
1 Dedicated CAS type - A-10 with all the bits the USAF want on theirs
Heavies:
C17s and Hercs
E3s
Off the shelf airbus tanker/transport
Helos:
Don't really know, can't even pretend to be an expert (although that doesn't usually stop people on PPRuNe) but:
Apache (prob better to let the army run them)
Chinook
Smaller thing - I have seen the Merlin in action and liked it but it seems to be a bit of a marmite thing.
UAVs
Whole range, smaller ones operated by army. Plenty of Reapers or Reaper+.
Superficial and very little thought gone into it, but hey there you go.