Best way to increase CV

First, if you are doing this purely for "investment reasons", forget it. The return you get out of the giveaways at higher levels is absolutely not worth the money you put in to reach such levels. Just buy yourself the games you want, it is less expensive.

That said, if you are doing this for the fun of it, or just because gaining levels is cool, or because you want to be nice to the SG community without spending too much money... Here is another tool to compare prices: https://www.steamprices.com/. For example, you could put your region in, select "Cheaper in Your Region", filter by "Discounts", and check if the games are not in the bundle list, so you would get full CV for them. Note that usually things that get discounted 95%+ do go into the bundle list, so go for the 90% discounts. Also take a look at the games themselves -- even when I do not have the game, I at least glance at the Metacritic score or the Steam reviews, so I give money to good developers in the process.

Now, do not get upset if your CV drops or if the rules change for any reason -- IMHO, seriously worrying about your CV is a bad use of your time. That said, if you want to see your level increase, we do appreciate the gifts we get in the process! :)

Cheers!