Battle rifle and jokehammer, if you are infiltrator, invest in the SAS-R, best rifle in game. Look up the weapon sheets, video reviews on youtube etc. trial the weapons and come to a decision yourself based on your play style.
Versatile? Pulsar LSW for VS, GD-22S for NC, and TMG-50 for TR. Same LMGs that LevelCap stated as the best all-rounders for each faction. While they are more or less average and underperform in some areas (NC6 Gauss SAW is a beast at range when tricked out, Orion is beastly in CQC) they have no real downsides and possess great reloading times for their overall purpose.