White-necked Jacobin is probably the most common.
In undergrad I would sometimes check the Mexican Peterson out of the library, the Jacobin was in the lower righthand corner of one of the hummingbird plates, one of the showiest birds in the book. While I've seen the jacobin a few times before, this was certainly the closest. Somehow I didn't manage a sharp photo of one showing the white collar on the back of the neck, most obvious in flight.
The female and young males are somewhat duller, though the scaly underparts aren't shown by many hummers.
The other hummingbird that on each trip confuses me is Rufous-tailed. It's the most common Costa Rican hummingbird in most of the human habited areas and is easy to ignore. This is the typical look of it.
However, when it hits the light it can look a lot brighter and start a person searching for more exciting birds it might be, rather than the more familiar bird they haven't studied as closely as they should.
At some angles the green just lights up.
Even having to resolved to study this bird early in the trip it still took a little getting used to in some lights. One more look at the more typical view (of a fairly dully billed bird.)