As my signature states, I have several armies just for the very same reason - to have choices for my friends to play so I have opponents without having to trek 45 minutes one-way to a game store. It also really opened up my eyes as to what all the other armies do, their playsyle, and weaknesses. I can honestly say that every army I own, I play actively (albiet Wood Elves which are still all brand-new unopened boxes). I just purchased my 12th army a couple days ago from BarterTown - a 2500+ Tomb Kings army which I am anxiously waiting for mr. postman to deliver.
The order that I purchased mine were: Bretonnians, Lizardmen (both came in the 5th ed. starter box), Dwarfs, Undead (which is now Vampire Counts), Orcs & Goblins, Dark Elves, High Elves, Ogre Kingdoms, Wood Elves, Empire, Skaven, and now Tomb Kings.
The funnest army for me has been Orcs and Goblins simply for the unpredictability of it. And because of that, there was no "gaming stress" of trying to control everything as you simply can't. Random rolls happen often in this army as well as animosity. Though it is quite fun IMO to picture the crazy actions and reactions of the army and it's opponents in my mind as it happens on the table. If you go into the game expecting fun and a "whatever happens" attitude.. this army is great to play. On top of that, they really do have a LOT of options and different builds.
Also, with the new 8th edition model releases for the Ogre Kingdoms, they really have a lot to offer on the battlefield too - especially compared to 7th edition games. The models are, imo, top notch. The fluff is great and the army can be pretty fun and competitive on the table too.
If you are thinking about Dwarfs, Empire, Dark Elves and Chaos Warriors - that's great too. Though I cannot comment on CW as I have not played with them.. the others are fun in their own right: Dwarfs are great at controlling your opponents magic phase and have great artillary and stalwart defences. Empire is the Jack-of-all -Trades in that they have everything available... and not really master of any; lots of variety, colors, more fluff choices than you can throw a zombie's arm. Dark Elves have a decent variety of usable units, strong fluff background, tough regen monster in the Hydra, and a good Dark Magic lore. Just keep in mind that all of these and Choas Warriors are rumoured to be coming up sometime this year and just into the next as new model releases and possible army book as well.
The main thing for advice that I can offer is look through the armies, check out the models and read their fluff. If you find one that you can like in both those areas, then give it a try. Perhaps even going to your local game store or GW and find someone playing that army and ask if you can watch.. or even look at their models not being played. Or.. even better - finding a person willing to let you play with them!
- Lord Cedric