I'm a soda guy with many favorite flavors and brands. This list will be more than three, sorry OP
I'm not a brand loyalist when it comes to cola. Coca-Cola and Pepsi are both good, as are most generic brands (Sam's Cola being a big generic favorite of mine)
Dr. Pepper and his Clone Army:
Dr. Pepper is good, but again I don't mind drinking the generic stuff (Dr. Thunder is amazing)
Mountain Dew & Clones:
Original Dew is a nice soda. Voltage, Code Red, Baja Blast, and Merry Mash-Up are also all great flavors. Again, okay with generic stuff (Mountain Lightning is a favorite)
Most fruit sodas:
Orangette and Grapette are great sodas (I shop at Walmart a lot).
Fanta's Blue Raspberry, Apple, Orange, and Strawberry sodas are also favorites.
Speaking of strawberry, Hannaford's generic strawberry soda is just about as good as Fanta.
I like fruit punch sodas, but they tend to leave a strange aftertaste.
When it comes to root beer, I'm a massive fan of one brand in particular: Barq's. Barq's has a bite to it that no other brand can replicate.
But if Barq's isn't available, I don't mind cracking open a can of the generic stuff (Again, Walmart's Great Value comes to the rescue).
I'm a big fan of Canada Dry and Schweppes.
Every holiday season (around Thanksgiving), we buy their cranberry ginger ale. That stuff is amazing!
Outside of sodas, I also like regular and flavored seltzer waters. I usually get either Adirondack or generic. I don't buy La Croix or Bubly because they're too expensive and I don't drink seltzer all the time.