Seeing as this is the MW2 section, I assume you mean MW2 mod?
I guess as a good mother I have to love my two favorite babies /

Terminator Invasion V2
Hide and seek
other than that I think gungame is probably one of the best, (if not the best) mods out there.
Roll the dice is a brilliant concept as well.
If I'd have to rank them it'd have to be like this:
1. Gungame: it's simple but effective, never bores
2. Terminator Invasion V2: Can be loads of fun, total chaos, all out war. After a few maps it gets a bit old tho.
3. Roll the dice: great concept, brings a lot of variation. I'd have to make an edit of it to remove some of the rolls maybe.
4. Hide and Seek: (my version, not the prop one) It's nothing special, and can actually be quite boring, but if you play it with the right people it can be a blast. I had some great laughs playing this.
There might be other great mods out there, but I haven't played them enough to really judge. I don't really like the mods that have excessive menu's for buying stuff etc.