Ethereum is not exactly an altcoin in the common sense. It has a totally new code. So yes, I'd say that it is a good altcoin since it is the most inventive one.
Most people think the bitcoin is the main coin, and all other coins are the altcoin. So the Ethereum is treated as altcoin.
Absolutely yes, any coin other than bitcoin whether they are developed from scratch or a new code are considered as an altcoin regardless of what they think.
I treat all other coins as the altcoin. There are some altcoins which are very useful in the transacations.