Event Shop : Ion Smg/Carbine/Pistols. they are a good balance of stats and pretty good in spar if you get a hang of it. oh and Ak74u, Cm901 are good alternatives too.
Auction Guns : Most are now banned from being auctioned but Mp7, Mtar X, Mp9, FOG and Dual Rafficas are beast. auction guns usually have a upper hand in better stats compared to shop guns.. but now that's not the case, ever since all guns have been nerfed.
Shop Guns : Gm4 (My personal favorite) , M4, Sten, FAL, Thompson and Famas.
Reward Guns : P2ks are good in spar if you get used to using them. and of course the spar reward guns are great like Vector, M4a3 and Mp5.