I think Becca Tobin is the one that's friends with most of the cast. I think every single one of them, honestly. Jenna, Amber, Chris and Kevin seem very close to this day; Dianna was also close to Amber; Heather and Naya, of course, and apparently the two are still friends with most of them too. The ones that I'd singled out are Lea and Darren. I think Lea was the only one at Darren's wedding, so... I think it's very telling, tho, that the two cast members that got by far the most songs and were pushed as main characters are the ones who everybody else seem to have distanced themselves from. The fact that Rachel and Blaine were insufferable, even for Glee levels, didn't help them. It's a fact that Lea was a diva and let the power got over her head, maybe Darren did the same?