If you are just starting, I would suggest creating a few apps and seeing which ones catch on and then focusing your efforts on those. Create apps quickly to test an idea, and then iterate quickly when improving an app. As for the type of apps, I believe ones that have decent growth and retention are best. I don't like apps that just focus on growth, because it is not too sustainable long term.
I've tried multiple forms of marketing.
1. Admob - Got users to the app, but the app did not retain those users and it was quite expensive to even get them to the app. Over $1 per user for me.
2. Tried facebook ads. Just a complete failure for me.
3. I cross promote my apps within my own apps. That does ok. I'm not exactly sure the exact amount of traffic it drives, but it probably accounts for 5% of the user base on the apps i promote.
4. Facebook, Twitter Sharing. This probably accounts for 5 - 10% of new downloads within an app.