Don't have the packs, but I have played a looottttt of Multiplayer so I'm familiar with the guns they offer. I'm basing this on Gold difficulty in MP which is slightly harder than Insanity. Also I'm unsure if the guns in SP have received the nerds/buffs they did in MP. That being said...

Groundside * Venom Shotgun - Pretty fun, but not really much reason to use it over something like the Geth Plasma Shotgun. Also not even the best shotgun in the pack. * Executioner Pistol - Good, but probably not better than the Talon or Paladin. Still, you'll get access to it right off the bat...just don't take it with you to Mars. * Kishock Harpoon Gun - Fun and decently powerful. Probably the best sniper you'll have until you get the Javelin. * Striker Assault Rifle -Pretty meh. * N7 Typhoon - VERY fun assault rifle, and is pretty powerful. Very heavy though. Probably the best gun to give to squadmate. * N7 Piranha - Extremely good shotgun, probably 2nd or 3rd best in the game. * Acolyte - A pistol that is pure sidearm. Will completely strip shields and barriers in 1 or 2 shots. Very useful, especially for sniping.

Firefight * M-90 Indra - Decent gun that's more like a battle rifle than a sniper rifle. * Krysae Sniper Rifle - If it's pre-nerf, it's one of the best guns in the game. If it's post-nerf, it's useless. Not sure which, hopefully someone knows. * Reegar Carbine - The absolute best shotgun, and one of the top 3 guns in the game. It melts enemies insanely fast. * Cerberus Harrier -Not only the best assault rifle, but the best gun in the game. It has ammo issues, but it melts everything. * Geth Plasma SMG - Not very good. It tickles enemies and not much else. * Blood Pack Punisher - Good SMG. It's no Hurricane but it'll do just fine. * Adas Anti-Synthetic Rifle - Meh. I imagine it's fantastic during Rannoch but other than that I don't see much use in using it over the Harrier.

So for an Infiltrator, probably go with Groundside. If you plan on replaying with a different class sometime, probably go with Firefight.