Ultimately you should choose an army that you like the look of. Don't succumb to popular Internet sentiment. If you don't like the models you won't want to paint the models and your enjoyment of the hobby will diminish.
I will also point out that you should be careful about looking for super powerful Internet lists. They don't last long.
What do I mean by this? Back in 2002 I began an Ultramarines army which I based off of the 5th Battle Company. Over time I have added different elements, but I still use my 5th company as the core of the lists that I use today ten years later. In the same time I have seen hundreds of "super," lists come and go. These lists are usually highly specialized and very expensive and even small changes to the game can greatly reduce the armies effectiveness.
Currently the 40k darling army is a Necron army known as Cron Air. It uses as many Necron fliers as possible. The problem is each Necron Flyer is some $50.00 dollars. This army will set you back a small fortune and their is no telling how long the list will be super.
I cannot stress highly enough to purchase a balanced army (which I will discuss later). It will be cheaper and will last you as long as you play in the hobby.
Now I will give a brief overview of the current armies, starting with 40k ones.
Space Marines: This is the bread and butter army. It is almost entirely plastic (fun to model, more variety, light, etc). Space Marines are tough, skilled, and well equipped. Space Marine armies are smaller in numbers and as such are cheaper to purchase. However because they are an elite army you will have to make tough choices with what to bring to a game.
Space Marine armies also allow many easy painting techniques to be used.
Here are some cool images to wet your whistle.
Blood Angels are more close combat oriented and favor use of Jump packs.
Regardless of type, all Space Marines are tough and effective in combat.
Next up we have
Necrons: Necrons are similar to Space Marines in that they are tough and resilient and effective in combat. They are essentially metal skeletons who want revenge against the universe. Necrons are currently very popular but are likely to be reduced in effectiveness.
Necron armies are also mostly plastic and can be effectively painted using the dry brushing method (which is both quick and easy).
Next up- More 40k armies and then Fantasy armies!