Conventional wisdom seems to direct us towards Haydn & earlyish Mozart, but judging by the repetoire you are working on, you have a lot more to choose from.
Personally, I would almost certainly start with the Stamitz F Major which is rarely heard and contemporary with Mozart, or perhaps the delightful Marlborough Variations by Vogler from the same era.
For the Romantics, I have heard Grieg and Saint Saens 2 mentioned several times here, but Martucci 2, Sterndale Bennett 4 or Stanford 2 make interesting alternatives and excessive demands on the pianist are not present.
There are too many "hardest" topics here so, I will only say that the Schytte is a bastard.
Thal
Thalbergmad, the very sad reality is that orchestras often times want to play the very standard repertoire, because of reasons I still don't fully comprehend. Even competitions usually give a list of certain concerti you can choose from to enter.
And I personally feel that one should learn a concerto with
every intention to attempt to play it with an orchestra (for students, probably by entering competitions)! Why else learn it? To perform only with a second piano? Also, later in life, people usually get orchestra opportunities not by knocking on the conductor's door and saying "I want to play THIS concerto," but by the conductor saying "Hey, so-and-so cancelled and they were supposed to perform this concerto. Do you know it?" or "Hey, we need a concerto and like your resume... What do you have?" and then they pick the one THEY want (often times being standard one), or something along those lines.
Believe me, I am all for these suggestions you chose, and I WISH GREATLY that orchestras would be more willing to play these other concerti!! And even though I'm not currently familiar with all of them, your taste is good enough for me to trust until I listen for myself.
BUT it still remains that orchestras almost always want the standards. It's a shame, because there is SO much good repertoire out there besides the standards. At least we can get away with it a whole lot more in chamber and solo playing...
That being said, for a STARTER concerto, although I personally prefer other ones, I suggest something standard. I am personally not a fan of Saint-Saens 2 (I like 4 and 5 much, much more) or Tchaikovsky's 1st.
You might like Ravel's Concerto in G.