Best grind free to play games ?

There are a lot of these I guess. What kind of game? MMORPG? How about TERA?

Have you checked out Warframe?

War Thunder
Grind for unlocking planes and tanks. Very much grind!

tf2, dota2, paladins

What about Path of Exile?

marvel heroes, aka grind lords of marvel.

Warframe is the king of grinding I think.

Crusaders of the Lost Idols.

http://store.steampowered.com/app/534500/

Tiger Knight: Empire War
it's not very grindy, but got some elements it's a long run for top tier soldiers and armor.
Game is epic, I really recommend it. It's like Mount & Blade with more strategy.

Do you have a mobile phone or emulator? Dragon Ball Dokkan Battle.
You grind for dozens of different things, many of them on their own.

guild wars 2, you can play for free, but isn't the full game, you can't use a lot of features, but still can be playable specially if you like do all by youself

Sakura Clicker

World of Tanks. Ignore the "Blitz" version that appeared on Steam the other day, tho.

If boats is more your thing. World of Warships is fine too.

They're both really grindy.

http://store.steampowered.com/app/534500/
I like this one very much

Mechwarrior Online? Plenty of grind in that. Takes weeks to earn enough for a new mech, and weeks more to earn enough faction standing for a mech bay to put it in.

God knows Eve is a fucking grind, fancy hauling all my shit around? They don't really do armour but I can promise a virtual beret when you're done :P

Dota 2, have grinded ~3k hours...

digimon masters is the most pay2win game I ever played but you can get cash items from other players by buying with in game currency
it will take you forever to get anything done in that game if you do it all the way free2play