That is a very vague question. Try harder next time. Which countries? Which exchanges? Real time, delayed, end of day? How many securities do you want to query? Streaming or snapshot? Consolidated or single exchange? Which app you use? Budget? There are quite a number listed here: https://www.elitetrader.com/et/resources/
Your question misses a number of details. Your choice of data provider would depend on your business requirements and budget. algoseek.com is a good starting point.
Use the search function. Use google. At the top of ET's home page is 'Resources' button. https://www.elitetrader.com/et/resources/categories/data-sets-and-feeds.19/
Well, it depends on your budget and the exact type of data you are looking for. For instance, if you're looking for historical US-centric market data, try algoseek.
Has algoseek improved? I checked it out a few years ago and thought the CME Level 1 data was pretty low quality. IIRC, > 70% of the quote messages were missing vs Reuters and some other benchmarks.