Spain isn't the best, unless you get lucky. But they are always at least mid-tier and can be really, really good.
First, as Spain, accept that your game will be unlike any other Civ. Spain (and arguably Polynesia) is the only "Explorer" civ. My initial Spain opener is 2 scouts, monument, and then typically a Trireme or 2 as well early on. You need to get out there and scour the world. The order of search is your nearby lands, then get a foot party out there while searching for the Great Barrier Reef like a madman with your trireme. 1000

in the Classical or Medieval area is SO powerful.
Even if you draw an empty area for Natural Wonders, you can be assured of always getting an extra 600

and 6 free

in the worst case scenario. Nothing to sneeze at - assuming you explore the whole map.
Spain fits a Patronage-heavy game as well. You stand a great chance of having a free 300-1200

within the first 50-70 turns. This lets you buy up prime allies and get big bonuses early. You should also meet more CSes because you are scouring the map, so you can cherry pick.
As for the UUs, they are pretty decent. Conquistador is awesome but very situational, and the Tercio makes a really solid counter to Elephants, Cossacks, Keshiks, etc. that run wild in that era. And they keep their +100% vs mounted.
Spain is fun in any game, and they are a beast on a favorable map. FYI, I typically play default settings, pangaea or continents, and Emperor level.