The
newest one is Legacy of the Void, and since November 14th 2017 it has been Free2Play, well aspects of it are. You can play the 1st campaign (Wings of Liberty) free of charge for everyone. If you already bought WOL then you get Heart of the Swarm campaign free now too. If you bought them both then you DONT get LOTV campaign sadly, you would still need to pay for that if you want to play the campaign but it is discounted if you have bought any of the previous campaign instalments.
ALL of the multiplayer is free irrespective of what you have/haven't bought so even if you have never paid anything you can play the ladder free of charge with all of the same stuff as everyone else. There used to be seperate ladders for different expansions but they did away with it and it's now ALL free2play ladder.
I play it every day, far too much to be honest and the ladder is still very well populated. I never wait for more than a few seconds for a ladder game unless its 5am or something like that and even then it's still very quick. I think the most I've waited for a 1v1 game is about a minute. It's still pretty big in terms of the competitive scene, in fact viewership in the last couple of years has actually gone up quite considerably since 2016/17. Lots of big tournaments to watch if you are into that kind of thing, lots of good stories at the moment with the first ever non Korean world championship in ANY iteration of Starcraft 1 or 2. It not CS:GO or LOL big obviously but its certainly doing not bad considering.
There is co-op as well which is really fun, and a different challenge as it's more fun and less soul destroyingly difficult
. You get get to try all the commanders for free until you hit level 5 and then need to pay to buy them to continue levlling up but you do get a few of them completely free so you can play them and see if you like the idea, and pick and choose which ones you want to buy if you like them. There is ZERO play to win mechanic, its all either cosmetic or different commanders which are no better/worse than the free ones, just different playstyles and are entirely optional.
It is undeniably hard as nails as a 1v1 game but even the mid to lower end of the ladder is very well populated, and the matchmaking works very well once you have about 25-30 games under your belt so that it can accurately work out who you should be up against. Each race also now has independetly trackeed MMR (matchmaking ranking) so you can play all 3 races at differing skill levels without worrying about hitting higher/lower ranked players because of your 'best' race.
With it being F2P it's worth giving it a bash and seeing if you might still like it.
) came out and it seemed even worse than before for balance & dynamic plays.