well, their strongest attack is their leap attack for knockdown by far, if we're talking melee, unless the enemy is really large.
anyways, packmasters don't get a huge amount of bio-e to spread around, so i personally am really big on efficiency for them. low cost with good benefits if at all possible. combat benefits are nice, but focus on stealth, survivability, and perception imo; however, getting some ranged attacks on at least some of them can be really good too.
so, specifically, i like:
advanced eyes: they're recon. this is not too expensive and gives a nice boost.
electromagnetic vision: only on one though. being able to spot metal is super useful. being able to identify that 'helpful technojacker' as a nex-android is even more useful.
enhanced sight: if you can afford it.
forked tongue: at least one, maybe more.
motion detection: this is huge. get it on as many as you can afford for the automatic dodge boost.
sensitive whiskers: not a huge priority, but you may find yourself fighting in smoke a lot and this doesn't cost much.
bio-force field: you'd love to have this on all of them, but you'll be lucky to afford it on even one for a normal packmaster.
chameleon skin: you want at least one. and you also want to upgrade it to a stealth field if you can.
reinforced exoskeleton: this should really apply to the hound's leap attack, check with your GM. either way it's low-cost increased toughness. you want this.
regeneration(enhanced): gorehounds take a long time to heal. this helps soooooo much. you need basic first, which is unfortunate because it does next to nothing. you could probably get away with not taking this if you have a force field on a given gorehound.
resistance to electricity: avoid this. why, you ask? well, go look at the main book, page 101. read what the electric weapons do. now ask yourself: who would use these weapons for the
damage? the only reason to pick it up is that you want something that has a prerequisite, and electrical weapons are probably not as good as your leap attack.
resistance to heat: low cost, resistance to plasma and such. you probably won't regret it.
stealth field: you wish you could afford it for everyone, but it costs a lot so you probably can't. still you want at least one, in my opinion.
circadian rhythms: you probably want at least one, potentially two, for long-term surveillance. but the prerequisite is expensive, and technically is absorbed when you upgrade to this... talk to your GM.
enhanced leaping ability: this basically turns the leap attack into a ranged attack. just buy the basic version, don't boost it.
enhanced neurological connections: you'd love to have this for everyone too, but it's expensive. it's also a prerequisite for some other cool stuff. get it if you can.
floating air bladder: you may want at least one, if only for the gills (or, depending on how your GM interprets it, just buy the gills without the upgrade, and buy it on most of your stuff). hiding underwater is helpful.
suction cups and gripping hairs: low cost ability to climb almost anything. pick it up.
leaping legs: you can buy this instead of enhanced leaping ability if you want the bonus damage.
gliding membranes: can be handy. put it on something with one of the jumping abilities, and you get yourself into the air from the ground no problem. pay attention to the low speed, though.
prehensile feet: can be helpful on at least one gorehound for urban areas. not for picking locks, of course, just for manipulating doors, levers, etc.
combat spurs: for your packmaster, if your GM will make the ability to slash hoses and cables useful.
reinforced body area: wherever your GM thinks the hound's leap attack comes from.
bio-e expulsion vents: for their excellent cost:benefit ratio, these are your primary ranged choice for the hounds. good range, respectable damage,
heat projector: it is really nice if you can afford it; this thing packs a real punch, has good range, and unlimited ammo.
organic rockets: it's handy to have some emergency AoE. long-range emergency AoE with indirect fire is even nicer. just pick up a few for special occasions, probably don't go all out.
omega blaster: with all your bonus bio-e, you might be able to afford it. it's really expensive, but then again, sometimes you just want a 10-foot diameter laser of doom. if you do get it, you'll probably want to make it super, omni, and mega. even before picking up a second one.
spore discharger: the basic version is low cost and gives infinite AoE attacks against robots. it would be nice to pick up the upgraded versions and even omni them, but that's nearly as affordable... anyways, it's nice to have the basic model on one or two hounds if you can afford it for taking out large groups of robots.
screamer grenades: the first one comes with free sound insulation. might be worth considering.
trench foot mines: a good low-cost explosive that can make it harder for someone to follow your trail. not every hound needs them, but i'd want to have at least one hound with a few.
from the I Am Legion book, the eye spy and the eye trigger may both be of interest to you, with the latter depending on how much artillery support you're likely to get
mantis blades are probably worth the cost, especially if your GM considers your packmaster blades to be bone blades (they probably aren't, but maybe they are!). bio-forcefield spheres cost less than regular forcefield, and as such are probably more worthwhile for a packmaster in spite of the drawbacks. you may enjoy a shrieker launcher and the shrieker targeting ability, and possibly spread it around all of your hounds; for example, if you have one with exceptional vision that can fire a shrieker into a target, and you and all your hounds have shrieker targeting, you can all enjoy perfect accuracy against that target