Join Date: Jun 2009
Location: In a cell for revealing sensitive information regarding the Inquisition
Hey, first off, welcome to the 30k scene!
A word of advice before I go any further. For me, more than 40k, 30k is about a balance of fluff and tabletop ability. For example, you get more people playing the non-top-tier legions because they love the fluff/models than I think you do in 40k. Equally, if you are hoping for some game breaking tactics you are unlikely to find them here. There are some legions that are better than others but none that are so good that you may as well pack up when someone brings them, like you would with Eldar or something. Overall, it's much more balanced and generally a more even-keel mindset than 40k I feel.
Now that that preamble is out of the way, let's talk legions. At the moment the ones that probably best fit your bill are loyalist legions, namely the Imperial Fists or the Ultramarines. Both have quite good shooting and the Templars/Suzerines are murderous psychos in combat. Both Dorn and Guilliman are ok Primarch, probably not the hardest to kill but also not baby harp seals either. Guilliman is probably better than Dorn but the Imperial Fists Legiones Astartes is probably better than the Ultramarines.
On the traitors side, the Alpha Legion are a popular choice since you have the option of 4 special rules including infiltrate or tank hunter that you can give to your whole army, which can make them pretty potent. Iron warriors are a solid choice too and with all Volkite Terror Squad the Night Lords aren't slouches either.
Basically there's a couple from each 'side' that fit your brief, mainly because apart from a few special units all the legions get access to the same things so most of their ability in combat/shooting come from any rules they have and even if you doesn't have one rule the other does, then they both still have the same guns. I would have a look at the fluff of the legions and which ones have model/a paint scheme you like and then go from there because, odds are, your legion can do what you want it too regardless of which one it is.