For Ground the best weapons are high damage, non rapid fire weapons, since the Borg adapt on a per shot basis.
Personally I use Sniper and Shotgun (Pulsewave). Been using them before the Borg even did their adaption thingy, but they work pretty well for me.
They do good damage, and allow me to do damage at both long and short range.
Blast Assault, Split and High Density Beam weapons are fine as well. Wide Beam and Compression pistols are okay, but they have a shorter range.
Dual Pistols and Miniguns arent very useful since the Borg will adapt to them after a single use of the secondary, give or take a shot or two.
As for the sets. I like the M.A.C.O. set on ground, you can never have too many shield heals, and it comes with a sniper rifle.
The M.A.C.O. sniper does good damage, but it's primary fire is a burst attack, making the Borg adapt quicker.
Once you get the Interegral Frequency Modulator it's nice, but before you have the whole set the burst fire is kinda annoying.
Whoever created the Omega Force set, I don't know what they were thinking.
The set bonuses are very nice and all, but the weapon is just silly. Rapid Fire weapons don't do crap against the Borg.
As for the Space sets. Well... They're pretty. That's about it.
Some parts of it are pretty nice. I love both the shields. And the MACO Deflector looks useful as well.
But the Engines as well as the set bonuses are simply worse than those of the Borg or Aegis, or even Breen if it weren't bugged, sets.