I'm really torn between which system to use if I ever have the time to run a tabletop Birthright game.

D&D is nice and the setting was made for it, but it lacks the realism I would prefer in a campaign full of politics and intrigue.

GURPS is very realistic and has a great combat system, but it creates some dark and dull atmosphere that I can't quite discern the cause for. My latest theory is that this is because of the system's lack of classes and special abilities. In D&D you level up after a couple of sessions and feel great about it because you have great new abilities to try out. It gives you something to look forward to. In GURPS you simply gain the shitty skill points, increase your skills at outrageous sp cost and still don't feel your character is much better at anything even though the campaign is close to an end.

Warhammer 2e is somewhere between D&D and GURPS in realism. The professions are very interesting. However, the combat is weird. You will miss a lot, when you hit you will hit hard, and when you are hit your will be hit hard. In an equal fight, you will be overwhelmed by a sense of helplessness as your roll d100 for 5th time without having scored a hit, just to have the battle determined by who gets lucky enough to get the first hit.

Warhammer 3e has a combat system that looks more like a board game. It is not that bad. The combat is very deadly, however, and the wounds can be quite crippling. Even when you win in combat, your party will probably have to pay a lot of gold or spend a lot of time to remove all the wounds.

I have never tried Fate. It looks interesting for some other settings, but I would like to play Birthright with a system like that.

My best choice for now would be a hybrid of D&D and GURPS which uses D&D races, classes, skills and feats but uses GRUPS combat system. Don't know if I will ever manage to develop such a hybrid. Seems more difficult than breeding a half-Elf, half-Dwarf.