This is a recurring question. So, instead of repeating it over and over, just point people to this editorial.

One-line summaries tend to radicalise people, but let's try anyway: great techs, great devs, great recognition.

CryptoNote

It uses CryptoNote technology, which, compared to most other coins, means:

higher privacy if implemented correctly (ring signature + stealth addresses).

higher scalability (no fixed limit, including the infamous 1MB blocksize, very high efficiency of the ed255199 signature scheme)

higher security (the Curve25519 is considered safe by cryptographers, contrary to secp256k1

changes are so fundamental that the adage "whatever coin X can do, Bitcoin can add it" doesn't apply here

emulates bank secrecy on demand and permits regulatory compliance (view key - see Monero, the Next Step, slide 9)

enforces net neutrality by design (miners cannot become censors, Wikileaks cannot have its donations delayed and a competitor cannot hinders payments to your company)

smooth emission curve (no block halving here)

GPU-resistance (GPU are only slightly faster than CPU). CryptoNight is said to be "memory hard" or "AES heavy"

Code for wallet is separated from code for node, which is more secure

Remarks:

Some altcoins exhibit one technology that Monero has: Shadowcash has cryptonote-like privacy (but only on exceptional situation called Shadow-to-Shadow), Vertcoin and some others have stealth addresses, etc.

Improper implementation could lead to no actual privacy. Best example is Bytecoin, where 80%+ of the outputs are owned by unknown parties; so unmasking which signature in a ring is the real one is trivial for the said unknown parties.

More technical information on CryptoNote: https://bitcointalk.org/index.php?topic=975984.msg10663673#msg10663673

Leader

The undisputed leader in Cryptonote-based coins:

Specific development

Monero has its own improvements that other CryptoNote-based coins ("altnotes") do not have:

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