Primarily Spain I think? I couldn't find a good source saying they would block the membership outright but there's an article here which includes some details about which states are opposed the EU negotiating potential membership for Scotland as part of Brexit (I realise that's slightly different to Scotland joining as an independent nation).
'Brexit: Spain and France oppose Scotland EU talks' - http://www.bbc.co.uk/news/uk-scotland-scotland-politics-3665...