Rank 1: Glock 17
Rank 7: Raging Bull
Rank 15: Desert Eagle
Rank 17: Use FAL if you must, but Deagle has better mobility. FAL gets better at rank 21 with Fast-Reload Assault Rifles and again at rank 24 when the Fitness skill is unlocked
Rank 25: If you are using Deagle but things are getting too tricky, switch to MP7 or FAL
Rank 29: AUG Para
Rank 33: Nitro Express (that's pretty much it until you hit elite ranks)
Rank 35: ARX is great and everything, but Nitro is still awesome too. Nitro saves you money, but if you must switch, ARX is a solid primary.
Rank 41: Still Nitro or ARX. Surprise! Minigun can help kill purge nests, but its ammo costs are too high to use it as a primary.
Rank 43: Nitro, with Mark II for heavy targets (Devs and Nests only, unless the train is absolutely massive and in the way of the nests)
Rank 44: Mac-10, with Mark II for heavy targets and Nitro for fun. Mac-10 does a better job of holding of spread-out swarm zombies and has slightly better mobility, Nitro works better against groups, especially when they include Mamushkas, Shadows, and Butchers
Rank 45: Seizure, from all I've heard, is epic. Mark II is still your best bet against heavy targets.
Rank 47: if SCMITR is non-premium (there are rumors, but it could be that the latest version just fixes the bug where you could buy it before reaching rank 47) then give it a whirl against heavy targets and huge crowds only.
Rank 49: if Wipeout is non-premium (there are rumors, but it could be that the latest version just fixes the bug where you could buy it before reaching rank 49) then give it a whirl against heavy targets only. Well, the ammo is cheap enough that you could shoot crowds too, but Wipeout is too OP for normal use.
Note: I do not recommend LMGs because of their ammo costs, but if you use the recommended weapons well you will have enough cash to buy them for fun. Keep in mind that you'll be slow and have excruciatingly long reload times. I do not recommend shotguns because their low mobility and utility mainly against crowds only makes them niche weapons, not primaries. With that said, after rank 18 (fast-reload shotguns) they are viable weapons, and after rank 39 (High-Capacity Magazines, which doubles your clips and is ridiculously good for small-clip weapons such as Nitro Express and shotguns) they can really clear a crowd. Some prefer Stoeger all of the way, but the later shotguns unlock later for a reason. Winchester's clip is its only real asset, SPAS has excellent damage as well, and the M1014 shoots fast, has a large clip, and has solid mobility.