A Google SERP API is what most teams need, and every provider still selling returns Google. The differences start the moment you need anything else: a Bing SERP API for a market where Bing matters, or Yandex for Russian-language results, Baidu for China and DuckDuckGo for a privacy-minded audience.
SerpApi is the broad one, with more than 100 search APIs behind one key, including Bing, Yandex, Baidu and DuckDuckGo, and Google verticals such as Maps, Jobs, Scholar and Trends. A SERP API Google Scholar request on SerpApi, or a Google Trends one, is the same call with a different engine parameter, billed as one search.
Bright Data sells its SERP scraper API as part of a scraping platform, and it returns Google, Bing, DuckDuckGo, Yandex and Baidu results, as JSON or HTML, at the same $1.50 per 1,000 whichever engine you ask. DataForSEO covers Google, Bing, Baidu, Yahoo and YouTube.
Serper is Google only, and is upfront about it: search, images, news, maps, places, videos, shopping, Scholar, patents and autocomplete, every one a Google vertical.
If the job is Google and nothing else, every provider here still selling can do it, and price decides; the moment a second engine is in scope, the shortlist is SerpApi, Bright Data and DataForSEO.