Which are the best competitors to Ethereum, and why do you think so?
(I know on top of my head that Cardano and IOTA try to challenge Ethereum,
although they still haven't proved much from my understanding?)
Currently, i don't think there is any other altcoin out there that can really compete with Ethereum's adoption rates.
If i absolutely had to choose, then Cardano and NEO could be possible options. But in the end, it really boils down to what the real world uses and that is Ethereum. Moreso when ETH finishes solving scalability issues, all other blockchain platforms will crumble.