I think there is merit in looking at this topic when dealing with orks and gaunts:How to kill GEQ
From that we get:
1. Storm guardian with flamers
2. Guardians with scatter laser
3. Swooping Hawks/Striking Scorpion (depends on if the target is in cover or not)
Another thing to thing about is the pure number of attacks(shooting and melee) the unit can deliver, comparred to their point cost, then we have some interesting contenders:
Swooping Hawks 2,17 points per attack (ppa) [no exarch]
Storm guardians with flamers 2,51 ppa
Defender guardians with scatter laser 2,88 ppa
Striking Scorpions 2,15 ppa [no exarch]
Dire Avengers 3,71 ppa [no exarch]
The reason I suggested Dire avengers is because they are good against first turn charges.
Since they were suggested I included these:
Warp Spiders 6 ppa [no exarch]
Windrider 4,6 ppa
Shining Spears 4,43 ppa [no exarch]
Howling Banshees 4,33 ppa [no exarch]
but they do not really match up.
Warp spiders are also actually worse against orks than gaunts, due to the 8th ed strength/toughness table. Their rending is even less useful than the shuriken rending, because these rendings only works fully against 4+ saves or better, and the spiders need at least power armour to get their full use of it.
This is why warp spiders are a terrible choice, but also makes the defender guardians and Dire Avengers less useful than against GEC.
Wraithlord with flamers and scatter lasers 7,26 ppa.