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The real issue is that neither one would attack the other. Watcher would only attack if threatened, and a Guardian would have no reason to attack a watcher.
However, if such a conflict did occur - It would be a close battle. Both have been around for eons, are Immortal, have vast energy, dimensional and matter manipulation. It might depend on which Actuality they are in, if it is in either of their respective Actualities I would give it to that person, otherwise I think tie.
"The Watcher wins but neither are great fighters"Lol that reminds me during the credits of MUA there are funny as phone calls going on, and they have one with Uatu speaking very calmly "It is clobbering time!". He says it so calmly and it is bloody funny.