The only gifs I use are ones that I look up on the fly. I mostly use them for RP accounts--things like ic inboxes or occasionally navigation posts in order to add a little flavor to them. I used to have a huge collection, but since I moved it to my external hard drive I've more or less lost interest in digging them back up.

I do use a lot of custom emotes on plurk, however! I mostly use Pusheen or other cat related gifs, with the occasional fandom emote thrown in for flavor and to fill the gaps. They're cute and fun to look at. They can be a nice way to cheer someone up as well; sometimes, getting a cute gif of kittens rolling around with each other or something to that effect can make a person smile when they're having a bad day. It's the same logic that would apply to youtube videos and memes.

I'm not sure that I could choose any favorites, but I have my collection of Pusheen plurk emotes here! I think they're all really cute.