What are the biggest events to follow for the most competitive decks in Magic the Gathering?
Now that TCGPlayer.com includes decklists from StarCityGames (SCG) and larger MTGO tournaments, they are my go-to site for decklists. Here is a search for the number 1 and 2 decks from all of the recent major Standard tournaments. 

In case the link rots, you can go to TCGPlayer.com, click on "Magic Decks", and search for the winning decks from major events. Right now it looks like they include:


Grand Prixs
Pro Tours
Pro Tour Qualifiers (including those on MTGO)
Magic Online Championship Series (MOCS)
SCG Classic/SCG Open


You can also view the MOCS decklists directly on the Wizards of the Coast website. The search functionality is pretty buggy. Searching for Standard format will yield no results in over two years. However, you can search for "Standard MOCS" without quotes to get at the more recent results.

StarCityGames.com also hosts their own decklists, which you can find here.