I used to use YouCompleteMe with Vim, then switched to deoplete on Neovim, because deoplete was easier to extend without having to maintain a fork and the user experience was pretty much the same.
I briefly tried out Coc but didn't find it to deliver anything compelling to make me switch away from a relatively fine-tuned deoplete-powered set-up (along with UltiSnips, LanguageClient-neovim, and javascript-typescript-stdio etc). I like the fact that there is competition though and an LSP ecosystem that's getting more and more robust all the time. We finally have multiple high-quality options to bring IDE-like features to Vim.
I used to use deoplete.nvim, LanguageClient-neovim with javascript-typescript-stdio, but they easily broken lots of time everyday and debug the problems is also quite challenging. So I made it to be able to load fork of vscode extension, and coc-tsserver implemented more than 95% features that vscode could provide. It's not easier to switch vim plugins, consider come back when you would like to try out some new features.
I briefly tried out Coc but didn't find it to deliver anything compelling to make me switch away from a relatively fine-tuned deoplete-powered set-up (along with UltiSnips, LanguageClient-neovim, and javascript-typescript-stdio etc). I like the fact that there is competition though and an LSP ecosystem that's getting more and more robust all the time. We finally have multiple high-quality options to bring IDE-like features to Vim.