TLDR: So, if you want to attack all the time, ideally playing and attacking quite a bit more than most people, probably go with Germany. If you want to be able to upgrade faster, generally have a somewhat stronger economy at any given point, maybe not play as often, maybe be more defensive, go with the Greeks. Though like I said I don't know how much heir discount is, but at least the effect should compound over time.
To be honest I haven't played these three. But for what it might be worth, the British bonus looks very week, and while they have a good unique unit, but no civ's unit is poor, so it shouldn't be the deciding factor.
The German rally speed up looks very nice, it has been used to really kick my butt more than a couple times. It can really move your troops around right where you want them for a lot of the battle, especially in later ages. I've had gunpowder or higher age players who seem to have their troops rallying, running extra fast, to exactly where they can do to most damage, for what feels like the majority of the fight. The problem is that the rally is only going to be worth a lot if you play and attack quite a lot, or certainly at least more than average/less active players seem to. I can see the rally being great for raiding attacks where you are really after resource buildings, the rally could help you avoid being hit by defense buildings, which in some cases should be able to somewhat reduce the number of units you need to use/pay for again, and help your units survive longer to steal more resources. In attacking to steamroll people and get tons of medals, it would still help, maybe just as much as when raiding, as it will help you kill defences quite a bit faster, letting your troops live longer, and destroy more of everything else. Germans are one of the best on offence. Decent unique unit, but again not a deciding factor.
The Greeks look very nice economy wise, though I don't know how big the construction cost discount is, which is the key point of this civ. The speed up might be nice very early on in the game, but will have already lost a lot of its point by the time you can choose a civ and even get the bonus, since building times are in hours, not minutes, by then, and will in later ages take many more hours, and for some upgrades and research, days. Thus, you normally probably won't even be playing when a building is done a few minutes sooner than other civs could do it, so that bonus will probably rarely affect players, aka, will rarely matter at all. If you like cavalry units the Greeks' is either best or second best in game (the other being French), though no one is going to use cavalry to be a major part of their dps. No matter what, cavalry, even unique cavalry, are about the lowest dps in game, the very worst for dps per unit capacity space I think. But Companions' moderately increased dps can mean they could defend themselves against smallish numbers of archers, and they could do a bit more damage to the defence buildings they attack. But then some people does use cavalry much at all, instead going only or nearly only for dps units. So their unit is either first to second best, or possibly as bad as worst to second worst, depending on personal preference and the situation in any given fight. Nut if you aren't attacking especially often, this shouldn't matter very much. It could even give you a reason toe experiment with having more cav, maybe using them in ways most people don't
Or hope that someone that has played these civs replies.
Just don't play Chinese. Their bonuses are useless at least 97% of the time, and not very significant when they occasionally could be used or noticed. Fine unit (higher dps archer), but not sure if it is really better than the British longbowmen.
