Considering the functional nature of ETH as a way to build Dapps, establish smart contracts, and as a platform for other offerings.
These are the platforms that seem to be its closest competitors,

NEO - Origins in China, earlier known as Antshares, NEO is the biggest 'Functional' platform. It already hosted some successful ICO's lately.

Waves - This is another platform, smaller than NEO, but is 'Functional' as well, its hosted some ICOs's as well recently.

EOS - This is a challenger; The promotor is a veteran of Bitshares & steemit , They have good funding and have also put out a testnet sometime in december if I'm not wrong. This will be good to watch.

ADA- Cardano - This is a new challenger Based in Japan. Not a lot of work has been completed, nor do they have a 'Functional' platform as of yet. There has been some news of its blockchain to be used for an educational credential usecase (being developed) by some Universities in Greece. They just have a wallet published (reviews say its a bit buggy atm) BUT they seem to be developing a very robust platform that is privacy AND regulator friendly (goodness knows what that may mean) BUT the promoter seems to have a lot of street cred, he contributed to the ETH project and Bitshares as well if I'm not mistaken.

Looking forward to hearing about other promising platforms.