For me:
1. Blucher, no need to ask why, haven't bought him though.
2. Murat, no need to ask why, bought him early, almost maxed him and inserted with a pistol and tent
3. Karl, duh, bought him, almost maxed him, inserted a ferguson and tent
4. Napoleon, best arty gen, haven't bought him though.
5. Mahmud, desert may be useless, but every other skill is good, bought and almost maxed him and inserted an art telescope and medkit.
6. Massena, cheap aura, bought him early, maxed, inserted with a sword and ambulance
7. Nelson, best navy admiral, bought him on the late game.
8. Lan/Dabrowski, lan for that damage, dabrowski for that aura, bought both.
9. Wellesley, people have underestimated him, he doesn't have aura, but he deals some excellent damage, haven't bought him though
10. John/Alexander, john for that double terrain, alexander for that aura, bought john because of his double terrain. Alexander is good too but i didnt buy him because john is cheaper with 50% discount.
Honorable mentions and why I didn't choose them:
Davout - quite expensive
Bismarck - quite expensive
Lannes - not for late game, still good tho
Sakurako - 1 admiral is needed, i already have nelson
Washington - the damage he deals is good, but karl and maybe massena is better, idk
Trevile- thought of buying him, but didnt
Hood - just like treville, you only need 1 admiral
Kutuzov - quite expensive
Paget - just like davout, but no since i already have 3 cav gens
Sophia - her palace ability is just too hard to give up
These are just opinions. Nelson and Karl are the only expensive generals that are worth it