If you want to know where they all are,
I wrote up a list here. Since then I've heard Piglet occasionally shows up outside Journey into Your Imagination with Figment, but haven't seen him myself.
If you want opinions on the best spots, I'd recommend Mary Poppins and Alice in the UK. It's not crowded around them so they tend to have one on one conversations if you want to or are wearing something that interests them. They also understand the concept of needing to be loud so people hear them (unlike certain other people who I won't point fingers at but it's totally Jasmine).
Also, Joy, who is the only "head character" I've seen really interact one on one, the others, sadly, tend to be too busy constantly moving slightly in another direction to pose to really acknowledge you. Winnie the Pooh is definitely also worth seeing because he tends to do less actual greeting and more just wandering around the Imagination Pavilion lawn being Pooh and it's really cute to watch.
(All recommendations subject to me just being flat out wrong or characters having "off days", Purple Pigment will not be held liable for any disappointing character meets.)
Edit: Oh gosh, and today Mickey returns to his old spot outside the short film festival! Results may vary on how good the new Sightings are (and you gotta have the mask), but it needs to be mentioned.