Which is the best bidding strategy in Google Adwords, is it manual or automated?

Manual CPC bidding is best if you have sufficient time to manage your PPC campaigns!

Or else run campaigns by keeping a specific target CPA and simultaneously look at the ROI also.

Thanks.

I'd say Manual when it comes to effective bidding strategy, gives you more control of your budget. 

100% manual. Automatic is good only if you're not an expert or you don't have time. If you have some time to juggle with numbers, manually optimizing bids will bring the best results.

Dario 

Both Types of bidding are useful, Manual bidding will be use full in bidding low ROI keywords, and automated bidding will be useful in bidding lower position and High CPA, Google Conversion rate keywords