Speaking from personal experience, "Phnom Penh's best Mexican" isn't saying much. Mediocre Mexican is more like it. But it is conveniently located, near the end of an alley off Street 19, behind the Royal University of Fine Arts (which itself is behind - coming from the quay - the National Museum), and next door to the Blue Lime hotel.
I might check it out when I visit in a couple of months, I like a good burger any happy burgers on the menu?. Is it close to street 51, which I have been advised to check out. Good luck with it and take care.
Odd-numbered streets, with a few exceptions, run more or less north-south. Numbers go up as you move west, but several numbers are skipped. After Sisowath Quay, the main through streets are 5, 13, 15, 19, Norodom Blvd., 51, 63, and then Monivong Blvd. From St. 154 south to Sihanouk Blvd., the first street in from Sisowath is Sothearos Blvd.