Pretty much anything will do performance-wise, so it really just depends how big. For my 2008 MacBook5,1, I'm using an ancient Kingston V+100. It was relatively slow for an SSD even when it first came out and so by today's standards it's really slow for an SSD. But it's still light years faster than a spinning hard drive. For my 2009 MacBookPro5,5 I'm using a Samsung EVO 850.
Ideally you should keep more than 20% empty though so keep that in mind when choosing the capacity, and remember to
enable TRIM. A brand new 250 GB Samsung 860 EVO is about US$90. Just beware though that the no-name Chinese brands (eg. King Dian 240 GB) have a reputation of dying prematurely, so don't get a bottom of the barrel no-name SSD.