I was very late to the Sims 3 party and only bought the game at the time of ITF, though I didn't buy that first. With the starter pack, I bought Island Paradise then bought the rest of the EPs and SPs gradually. Isla Paradiso is still my 'go to' world because I just love the water based activities and the tropical/coastal build sets, and houseboats are so enjoyable to build and to sail around the islands. It's a royal pain in the butt to rectify the issues causing *major* lag there, but after following a list of things to 'fix' I then saved the world as a master copy. Now all I do is load the master and make a new save for that family. It works really well and I have many many different games in IP, it's well worth the effort to make it right!
I have all store worlds too and love them for different reasons, but seem to play more in Lucky Palms, Sunlit Tides and Aurora Skies.
As for custom worlds, I can recommend any of Rflong's masterpieces, they are truly outstanding, but I especially love Beach City and Legacy Island III.
I also love some of mysimrealty's worlds on her website, such as Meadow Glen, Deery Meadows and Storybrook County.
For tomb lovers, simsample's world called Jericho is just amazing. Some of the most cleverly thought out tombs with outstanding decoration, a real pleasure to explore them all.
Another world on my list is Enekjaer by fagersims. It's a very picturesque world, especially in winter with the snow. IronSeagull on youtube made a video about it, go check it out! :)
Duneoog and Isle of the Midnight Sun world are small, fast loading - google them for links. The Isle of the midnight sun challenge is an interesting way to play, it's listed on modthesims website.
Johnnywr has a few worlds that are outstanding, beautiful masterpieces - Great Bear, Cape Garner and Mariner's Reach are all atmospheric and gorgeous to play in, especially good for picture taking.