In my opinion, Spanish is the overall winner here. I think the Romance languages all share quite a lot of similarities. I believe more so than the Germanic languages for example. And I think Spanish has the highest leverage out of the Romance languages because about 80% of the vocabulary is similar to Portuguese plus the grammar also has a lot of similarities, then you have languages like Galician which is in between Portuguese and Spanish, so even closer to Spanish than Portuguese, and then Catalan which is more between Spanish and French, so Spanish also provides more leverage than for example Portuguese or Italian here. Italian is also fairly close and although it is not as close to Spanish as the other languages I mentioned, it is probably still closer to Spanish than it is to French or Portuguese. Then there is Occitan which I think is between Catalan and French but closer to Catalan. Then French is a bit different from Spanish, but it’s not really further away from Spanish than from the other Romance main languages (Catalan and Occitan are, of course, closer as I said). The language that differs the most is Romanian I think, but knowing any other Romance language will definitely help with Romanian and still allow you to catch a few words without knowing it.
So, to sum up, I believe Spanish gives you a good start to learn at least 7 other languages much more easily and you will also understand quite a bit in these languages even when you don’t speak them, you will probably be able to read between 70 and 80% in 3 of these languages (Portuguese, Galician and Catalan) without learning them. Listening may be a different issue though, as it can be challenging with the different pronunciation when people speak fast. And I didn’t even mention the other Romance minority languages (Ladino, Romansh etc.) which probably share many similarities as well. I just don’t know all that much about them. Of course, any Romance language will give you a good start to learn the others, I just think Spanish shares similarities with most of them and is kind of in the middle.
On top of all that, Spanish is also one of the most spoken languages in the world. With Spanish, Portuguese and French + English you can even communicate anywhere in North and South America as well as in Africa plus quite a few countries in Europe.