Oh my, this IS a tough one, I've been a sucker for team books all my life; the X-Men, the New Mutants, the Legion of Super-Heroes, the JLA, the JSA (Who I mostly knew about through the JLA/JSA crossovers), the All-Star Squadron, Grant Morrison's Doom Patrol, the Batman Family (which I knew when it was at the more ... manageable scale of one Batman, one Batgirl, one Robin and then a replacement Robin and a Nightwing... not that I didn't like the additions, but...) and of course, the New Teen Titans. I'd even count the five principle Bronze Saints from Saint Seiya as a team.
All these have been part of my comic reading experience since the age of... well let's not go there, so picking one is like being asked to pick between a boxful of cute, but different puppies. It's almost impossible.
But, whilst I've posted a LOT of New Mutants and New Teen Titans and Batman Family stuff over the years, there was always one team which was THE team... and it might not be one you'd think of, but it holds a special place in my heart.
( The Legion of Super-Heroes )