A guild member of mine is leveling quite rapidly in Rkindaleft. If you have Orsinium DLC, try there.

There are a couple of guides on the internet that give tips on which quests to do and which to ignore when leveling quickly to 50.

I followed the advice of one of them a couple of months ago and it took me two weeks of not very serious, no grinding involved playing.

Now that the toon is at 50 I am going through all the zones one by one and completing the smaller quests (for good XP when enlightened) and getting the shards that I missed while speed leveling.

Do the main quest and lots of random dungeons. Go to Wrothgar and do lots of those quests while farming delves for short periods of time. Start DB and TG. Use experience scrolls or potions.

If it's an alt then craft him blue training set gear every 15-20 levels or so. Don't forget that you can use these training sets on any and all alts you make in the future so don't deconstruct them when you've out leveled them.

The fastest way to get to level 50 would be grinding. I wrote a guide on how to do it here which goes into detail on the areas for each faction and the buffs that make it faster. The short version is get training gear (Seducer for Magika, and anything that gives Stamina regen for stam builds so you don't run out of resources), Regen food, ESO Plus, Crown Scrolls or Ambrosia's and Ring of Mara if you have a friend . Then go to places like the Orsinium public dugeons (they can be packed with people sometimes) or your factions zones with are usually empty and kill everything. When I did this I got my Templar to level 50 in 15 hours of playtime which took two and a half days.