I agree with the posters who said that both favorites and must avoids are largely matters of personal preference. That being said, here are some of my personal favorites, the dives I want to do every trip.
Palancar Caves/Palancar Bricks. There is a longish, downward sloping swimthrough on Palancar Bricks that exits at about 95', right at the wall; you can look down into the abyss. Then you make a right turn and swim along a wall punctuated by canyons rising up to the top of the reef. I love that dive. And I love the soaring coral structures on Palancar Caves.
Columbia Deep/Columbia Shallows. On trips when I haven't been diving in a while, my first day easy dives of choice are Columbia Shallows and Palancar Gardens - both relatively shallow, both typically easy current (though that can vary day to day), both beautiful dives with lots of sea life. Columbia Deep is just as beautiful, a bit more challenging.
Santa Rosa Wall. I love the feeling of soaring over the abyss, and if you are lucky enough to see one or more majestic spotted eagle rays coming up the wall, well that is amazing. NOT for divers who can't control their bouyancy or don't pay attention to their depth; I've seen divers on what was supposed to be a max 100' dive drift down to 135' or 140', completely oblivious to where they were. Enjoy Santa Rosa, but PAY ATTENTION, please.
Tormentos. The current here can get fast, but there's usually a lot to see, especially big green morays. This dive is always fun for me.
Dives that I would avoid include Barracuda - I've been on Barracuda on 3 dives, and can honestly say that I enjoyed the dive just once. The first time was long, long ago, not very long after I got certified, and the current was ripping so fast that I was frightened of being swept out to sea. I called the dive and got back on the boat. The second time, I was much more experienced, over my fear of being swept out to sea, and I finished the dive - it was less a dive than a ride at Disney World - Mr. Toad's Wild Ride. Now if you adore scary rollercoasters, you might like Barracuda. If not, best to avoid.
I also dislike the Wreck Dive, C-53. Just boring. I'd much rather see soaring spires of coral and vibrant sea life than rusting hulks of metal.
Paradise reef during the day. I've had some wonderful night dives on Paradise, when the squid come out, the big lobsters are roaming the sand bottoms, and all the night life is out. But during the day, Paradise is just meh.