There are three ways to get heat into a margarita and they taste completely different. Muddle fresh jalapeno and you get green, vegetal heat. Use a chilli syrup and you get sweet heat. Use a chilli-infused tequila and the heat is already built in, evenly, every time.

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The one thing that matters
Taste before you pour. Chilli heat in alcohol builds slowly and lands later than chilli heat in food, so a drink that seems mild in the shaker can be fierce by the third sip.
Use a jalapeno-infused tequila, or muddle two or three slices of fresh jalapeno in the shaker before adding the rest. Two ounces of tequila, one of orange liqueur, one of fresh lime, and a chilli salt rim.
An infused one gives even heat every time. Tanteo makes jalapeno and chipotle versions, and 1800 does a cucumber and jalapeno. Chipotle brings smoke as well as heat, which is a different drink again.
Float the chilli on top instead of muddling it in, so you can lift it out. Adding more lime will not help; adding a quarter ounce more agave syrup will, because sugar mutes capsaicin.