I have played Tau and Guard, as well as several other armies along the way (Orks, Dark Angels, Space Wolves, Salamanders, Inquisition, etc). I grew tired of Tau very quickly. They are rather one sided as has already been mentioned. My favorite army both to paint and play with is Guard. I like them simply because the model range is so wide. You have at least 7 different types of models that I can think of, no make it 8. You have your standard Cadian and Catachan armies sure. Throw in some Vostroyan, Steel Legion. Tallarn, Valhallan, Elysian, and the Death Korp of Krieg and your army is more varied than any other type out there. Also consider the infinite number of conversions you can make by adding in bits from the Mauraders of Chaos box (among others) from Warhammer Fantasy, the massive number of tanks, flyers, super heavies, titans, and elite slot models, and the huge array of alternative models available around the internet and you have an army that not only fits the fluff (as many different planets forces are brought together to achieve an army capable of taking something) but many different painting challenges. My personal guard collection includes cadians, catachans, and vostroyans as the bulk of it. They all look very different but the paint scheme brings them all together. I include lots of leman russ variants, basalisks, chimeras, and several super heavies. I have half a dozen or so flyers and a warhound that's coming up as my next big project. I have 3 squads of Ogryns, 3 squads of storm troopers, some coverted fantasy models that act as psykers (flagellants), some penal legion troops (Mauraders and catachan bits), and lots of character type models that all offer unique painting challenges. Camouflage is as difficult as you want to make it and can fit any environment (I chose an urban environment). Tactics wise I have found that you can run a lot of different lists with guard and have success, sometimes more so than others depending on the opponents you play in your area. I tend to focus on the Armor 12 scheme with lots of tanks and chimeras or the infantry horde. It's always a nice feeling to put out more models than even an Ork army can muster and still win. Not to mention the infinite motivation you can find in movies and on television for painting a mere human in a militaristic future.
That's my recommendation. I liked converting with Orks but grew tired of painting them. I made an attempt at Eldar in 5th edition, but it didn't fly. I didn't enjoy painting the models (and that's my preference) and that's pretty much where Marines fall now too. Ultimately I recommend choosing something you find visually appealing to your painting techniques and go.