First off, you're generally best off with an army that speaks to you, so good on that
Assuming you're looking to fill a basic point total, the really short version is that Grey Knights and Chaos Space Marines are elite forces of fewer models with more asskicking/model, the Necrons and Tau are in the middle in how many you'll likely need, and Orks and Tyranids tend to want a LOT of models on the table. Mind you, for most the basic FOC (1 HQ, 2 Troops) will be 21 models or so.
Slightly longer for playstyle and gimmicks, but still short and I'm sure people that play them will speak up and correct me
Necrons are something of an odd duck because their basic weapon will glance any vehicle to death, and they have a habit of not actually being dead themselves. Can field something decently competent in every role you want.
CSM are Space Marines with some different tricks. No access to some of the toys the loyalists get to play with, but you get other toys and a remarkably easy time getting other armies allied to yours (don't ask me WHY they trust you. I don't know!)
Orks Orks Orks Orks... Lots and lots of boyz, because your saves are bad. Very melee-focused, shooting not worth much though there ARE some interesting options for it (one of the few reasons to ever read the Artillery rules).
Tyranids are probably most capable of flooding the board with models, though there's also a lot of (expensive) high-Toughness beasties in there that can't be easily gibbed.
Tau are the other side of the coin. Shooty as all hell, horrible in melee. Take the shooty gimmick perhaps a little too far, esp with some of their formations.
Grey Knights would need the fewest models just because they're small squads of very point-heavy (20+ points each) models. Very elite force. Everything that walks has some sort of Psyker rating to go with it. Army overall is mid-to-short ranged, you can outfight most things on a model-to-model basis but you're going to feel every loss. Troops start at 5 models.