It can be any game. It's actually up to you. I'm not an expert or anything, I do not have a channel, but I think newer games might garner attention. Also, do ensure that you are going to be regular in posting videos and that you have good software and/or hardware for editing them. All the best for your channel!
<~Trent (from this place) says the Wigglytuff circus above leads you to a nuzlockle run and credits Haychel for the avatar!