I've used most of the Thermal sights from MW2, which carry over to MW3.
Some are really good, depends on the zoom you want.
A short 2.5x, 4.0x, 5.5x, 6 or 7 etc.
Remember the bigger the zoom, the bigger the glint.
I used Thermal for most of my longshots for Orion camo.
1000+ longshots or something, can't remember but it was a lot.
Some of the best ones were.
SZ Holotherm (for shorter ranges).
Drexsom Prime 90, Teplo-OP 3.
There are others I can't remember the name. If I see any when I'm next on I will recommend.
Avoid the ones with a white lens, they tend to be harder to see especially with smoke.
Always go for the greenish/orangey ones.
If you go for a 2.5x Thermal you won't get a glint.
I think it's under 3.5x won't give you any.
But you obviously won't get the range potential.