Mine is probably dogmeat in Fo4 and lydia from skyrim. and Cait from Fo4 aswell.
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Another vote for Serana She was the only companion in Skyrim I enjoyed travelling with... Other than Cicero, of course. But he gets annoying after awhile I hate having a follower in Fallout 4 though. I like not to be seen. They are not master of how not to be seen. And dogmeat is ALWAYS IN MY WAY! Except when I'm overencombered, then he's 3 blocks ahead of me and still running.