best hero for getting to late game?

Earthshaker and Lion are both good picks for this situations, as they are perfectly capable of Position 5 super hard supporting. When with a team of carries, they usually need crowd control, and the most reliable form of crowd control is stuns and hexes. ES is much better in mid and late game high stakes teamfights, while Lion controls lanes better and is less item dependent. Also, a tip. If you're spending everything on being Position 6 support on a Dagger reliant hero, just make a Force Staff, it builds easier, it's still more likely to snowball you, and the fact that you'll die less will improve your money in the bank to get up to a dagger.

For solo supporting 4 heroes that lack early game, Undying is also a good choice, as once he gets Arcane Boots, he can get the team through midgame fights, assuming the enemies didn't pick Drow, Clinkz, Sniper, or Shadow Fiend, as those guys will waste your tombstone without any effort. He lacks disables, but is a very strong hero in early phases, which might just be what your team needs.

Vengeful Spirit is a good pick too. She's not great in teamfights, but she's an excellent ganker and she has this great ability to easily stack 2 camps at once, which can propel your team ahead. Also, late game Venge with 4 carries is really strong.

Dark Seer can be played as a support (shocking, I know), and he is effective at zoning out and harassing opponents with Ion Shell. Surge also works well through much of the game to help the team go into gank mode. Hero can also jungle and get his early due.

Wraith King is also an option, albeit a much riskier one. The hero can roam lanes or jungle or pull. What keeps Wraith King viable is him being extremely resistant to burst damage, making Reincarnation one of the best "Hail mary" skills in order to win the key teamfight and get the comeback gold. Wraith King plus carries is also insanely strong late game, shining among late game position 2-5 heroes.