Hard to say really, everyone is different in what they like. For example, me and my friends really enjoy Mount and Blade, but everyone else on my course think it sucks, and prefers to play Halo, or Call of Duty. It all really depends, You could take a look at a few sites, that may help you get a better idea.
From what I can see, First Person Shooters and WarCraft/Dota based games are the more interested clients, but, really you'd want to be someone introducing a new style to the playing field.
Of course, off note, you said simple, so I guess platform browser games would fit this category, quick and easy to play, casual, dont require you spend weeks on end playing it. A nice little start for a new developer, you could always try Social games for facebook, if you decide thats a good route.
And not to nitpick here, but should this be in the game design section or something similar ?
'Knowledge isn't key, but understanding...'
My qualifcations are not here to showcase, but for those I answer and ask, to get a better idea on my knowledge.
BCS Level 2 Certificate for IT Users (ECDL Part 2) OCR Level 2 National Award in Business Level 2 First Diploma in Media Level 3 Diploma in Games Design and Development Extended BSc Hons in Computer Games Programming (Current - 1st Year)
Let's ask a similar kind of question: "I want to make a house that everyone likes and I have a hammer, what kind of house should I make?"
First you'll need to check your environment or even pick where you want to start building, what kind of people are living here? What other kind of houses are build here? Are there living plenty of people here?
So you did a little research and you found the best spot to start building your house or maybe even multiple potential spots. Pick the spot that best fits your needs (getting people to buy your house) and then you start working on building your house!
You got your blueprint and your hammer ready! But perhaps a hammer is not the best tool for you to use in this case, be flexible and be open for using other tools, like a saw! Now start building that awesome house and throw it in the market!
Moral: Do your research, theark already pointed out some stats. To give you a nudge in a good direction (I'm not saying it's the best direction): Mobile games grow pretty fast and are easy to step in. (that's picking the best spot) but perhaps C++ isn't the best tool to work in the mobile environment, so perhaps dig in some new languages. (be flexible in your tools). And addicting is also a bit of research, a lot of people play(ed) angry birds, the concept is old, but still people picked it over dozens of other options that were mechanic wise the same, why is that? Look at some of the top games out there, what do they have in common? What kind of mechanics are they using?
It all boils down to research really, like theark also already said, tastes differ, so research is probably the way you want to go first, figure out what platform has the largest demographic and see what the games they play have in common. Perhaps that will give you directions in what you can make to make your game addicting to them.
This is not the golden road to success, mind you. But if you can't design addicting mechanics yourself, learn from others!