What's my favourite part of Cuba? Wow, that's a tough one.
Fully 100% of my trips have been to, or passed through Havana. It's got to be one of my favourite places in Cuba, but mainly that's because of the great friends I have there. I also simply love to wander the many neighbourhoods and small cafe's.
Other areas I've enjoyed have been the Isla de la Juventud which has a character all of it's own. It's very quite, charming in it's own way, and a place to just totally get lost (if that's possible on a 50 x 40 kilometer island). But there are no real "resorts" per se other than the 3 star Hotel Colony which is located out in the middle o nowhere. It's a haven for Scuba divers, but beyond that you must be pretty good at amusing yourself. I have enjoyed many days simply exploring the backroads of the island on a rented scooter.
Cayo Largo was nice if you just wanted to relax and do nothing, which is exactly what I did. Nice to just head up the main island road and walk for miles, then walk back along the beaches. Very tranquil and the best beach in Cuba.
Varadero was Varadero. Somewhat Kitschy for my liking and while it's indeed in Cuba, it doesn't represent to me the real soul of Cuba. It's sort of like the difference between going to a fresh butcher's for your steak, or buying is pre-packaged, shrink wrapped, and on a styrofoam tray. Same beef but wrapped and sanitized. Not really for me.
Trinidad was a lovely place to photograph and stay for a week. No idea what the beach resorts are like in that area, but it might be worth an additional week to further expore the area. I want to rent a scooter next time to explore further afield.
Hope this answers your questions from my perspective.
Steve