Informal:
* Puer comme un cochon: Literally "smell like a pig," this is a very direct and informal way to say it.
* Pu(er) comme un porc: Similar to the above, this is also very informal and uses a different word for pig.
* Sent(ir) mauvais comme un cochon: "Smell bad like a pig," this is a bit more nuanced than the previous two.
More polite:
* Avoir une odeur forte: "To have a strong smell" is a more polite way to say it, but it doesn't specifically mention the pig comparison.
* Sent(ir) un peu fort: "To smell a bit strong" is another polite option, but it doesn't explicitly mention the pig comparison.
Figurative:
* Pu(er) comme une décharge: "To stink like a landfill" is a figurative expression that conveys a strong sense of unpleasant smell.
The best option for you will depend on the context and your relationship with the person you are speaking to.
It's important to note that using any of these expressions, especially the informal ones, can be considered rude depending on the situation.