In my humble opinion, GW2 is still the best MMO in the market at the moment, even with competitors like Wildstar rising up in popularity with some audiences (mainly hardcore raiders). It's completely free-to-play after the initial purchase, with only vanity and some convenience items (endless harvesting tools etc.) available in a gemstore, with gems available to purchase via ingame currency as well.
Great freedom for exploration both on land and underwater with many huge and beautiful zones complete with vistas, skill challenges and jumping puzzles. Decent storyline to follow, but otherwise no linear route to take in zones while levelling, with freedom to choose even lower-leveled zones to entertain oneself in as level scaling keeps all zones interesting even at higher character levels. Most of the things you do in the game gives your experience (events, jumping puzzles, exploring new locations, crafting etc.) so you don't have to grind quests to reach the level cap. In fact, there aren't any quests in the traditional fashion as they're replaced by dynamic events and "hearts" scattered around the map. Many people have gotten their characters from lvl 2 to max with just crafting after collecting the materials with other characters, or buying them from the trading post.
New content added regularly with festivals for different occasions around the year, evolving storyline chapters every few weeks (mostly) and feature updates a few times a year (at least for the past year or so). Dungeons and Fractals have already been mentioned by other posters so I probably don't have to delve into those as I don't do them regularly.
Best combat system in the MMO market today, at least in my opinion, with highly mobile encounters, especially on several (world-) bosses. No trinity which means that you won't be spending hours looking for a healer or a tank for dungeons if you're interested in that part of the game. Most people seem to be focusing on damage, but many classes can also feasibly fill a hybrid support role all the while doing damage with the rest of the team too, and it can be important to have some support on most of the instanced content.