I know I can't be the only player here who enjoys loading up a capital ship and flying headfirst into the dumbest yet most heavily armed npcs around.
The only question was where to find these missions.
So, because I wasn't able to find the info on the wiki or searching the forums I decided to dig into the data files and put together a list of the highest difficulty missions available.
The difficulty rating number is not the same thing as the number displayed in-game and can be understood with the contents of diff2money.ini from \data\randommissions
The information on the missions a base has can be found in \data\missions\mbases.ini and the internal names for bases/systems in the \ioncross folder.
This list just puts the information in an easier to read format.
An example: The base with the highest difficulty missions is Fort McMurray in Alberta where the difficulty range is set at 95 - 100 (5,500,000 - 6,000,000 credits)
Liberty Navy + L.S.F.
[Li05_01_Base] 10 - 53 - Prison Station Mitchell, Alaska = LSF, Liberty Navy
[Li05_02_Base] 10 - 53 - Prison Station Fairbanks, Alaska = LSF, Liberty Navy
[Li05_03_Base] 10 - 53 - Juneau Shipyard, Alaska = Liberty Navy, BHG
[Li05_05_Base] 10 - 53 - Kodiak Research Station, Alaska = LSF, Liberty Navy
[LI14_01_Base] 15 - 53 - Battleship Concord, New Hampshire = Liberty Navy
[LI14_02_Base] 15 - 53 - Battleship Whitefield, New Hampshire = Liberty Navy
The Order
[Li05_04_Base] 10 - 53 - Battleship Atum, Alaska = The Order
[ST07_01_Base] 15 - 60 - Meskhenet Station, Omicron Zeta = The Order
[ST08_01_Base] 15 - 60 - Battleship Thebes, Omicron Mu = The Order
[ST08_02_Base] 15 - 60 - Taba Border Station, Omicron Mu = The Order
[ST08_03_Base] 15 - 60 - Planet Akabat, Omicron Mu = The Order
Gallic Systems [Way more missions than regular sirius ]
[Bw10_05_Base] 28 - 100 - Battleship Carcassonne, Tau-31 = Gallic Royal Navy
Orkney System
[BR08_04_Base] 53 - 100 - Battleship La Riche, Orkney = Gallic Royal Navy
[BR08_05_Base] 15 - 100 - Fort Albi, Orkney = Gallic Royal Navy
[BR08_01_Base] 15 - 100 - Reunion Wreck Cordes Outpost, Orkney = Gallic Royal Navy
[Bw08_04_Base] 53 - 100 - Battleship La Riche, Orkney = Gallic Royal Navy
Languedoc System
[EW13_01_Base] 15 - 100 - Planet Quillan, Languedoc = ga police ga navy
[EW13_02_Base] 28 - 100 - Nimes Station, Languedoc = ga navy
[EW13_03_Base] 15 - 100 - Montpellier Guard Station, Languedoc = ga navy
[EW13_04_Base] 28 - 100 - Narbonne Base, Languedoc = brigands
[EW13_05_Base] 28 - 100 - Limoux Base, Languedoc = maquis
[EW13_06_Base] 15 - 100 - Lodeve Space Port, Languedoc = gjunkers
[EW13_07_Base] 28 - 100 - Battleship Montmorency, Languedoc = ga navy
Dauphine System
[EW14_01_Base] 28 - 100 - Valence Base, Dauphine = brigands
[EW14_02_Base] 28 - 100 - Montelimar Base, Dauphine = union corse
[EW14_03_Base] 28 - 100 - Briancon Station, Dauphine = EFL, ga police
[EW14_04_Base] 28 - 100 - Gap Station, Dauphine = ga navy, ga police
[EW14_05_Base] 53 - 100 - Battleship Ambazac, Dauphine = ga navy
[EW14_06_Base] 53 - 100 - Battleship Battleship Embrun, Dauphine = ga navy
[EW14_07_Base] 15 - 100 - Grenoble Depot, Dauphine = maqis, corse
Burgundy System
[EW15_01_Base] 28 - 100 - Planet Nevers, Burgundy = ga police
[EW15_02_Base] 28 - 100 - Alesia Station, Burgundy = GMS, ga po, IDF
[EW15_03_Base] 53 - 100 - Montbard Base, Burgundy = brigands, maquis
[EW15_04_Base] 53 - 100 - Avallon Station, Burgundy = ga navy
[EW15_05_Base] 28 - 100 - Dijon Station, Burgundy = ga police, GMS
[EW15_06_Base] 53 - 100 - Macon Base, Burgundy = council, maquis
[EW15_07_Base] 53 - 100 - Bourbon Base, Burgundy = union corse
[EW15_08_Base] 28 - 100 - Bourg-en-Bresse Space Port, Burgundy = ga junker
[EW15_09_Base] 53 - 100 - Battleship Beuvray, Burgundy = ga navy
[EW15_10_Base] 53 - 100 - Battleship Rocroi, Burgundy = ga navy