Tau are one of my favourites.
A lot of the negativity towards them (Where I play, at least.) Stems from a dislike of the Gun Wall. (Or Tau-gis Defence Line). As stated before, simply deploying well, and pummeling targets at range, while not moving. Makes for a static game. Potentially boring. But also frustrating. Since a well played Tau Army will kill any melle troops long before they actually get useful. So the Tau's enemies spend the whole game running forwards and dying. Not Fun.
However, Tau can be highly mobile, if you chose to play them that way.
The Crisis and Stealth Suits have Jump Packs. Meaning that they get an extra random-distance move, at the end of their turn. (Which other armies either can't get, or get very few of.)
Also, their skimmers (Floating Tanks) are able to navigate the sort of terrain which would usually prove hazardous, without risk. Letting them safely land in a forest or suchlike. Giving more options for movement than other tanks.
If you ask me what the most static army is, I'd answer with Guardsmen. "I can't be bothered to actually move those 300 hundred soldiers."
If you want advice on which army you should start with on Table Top, then as much as I'd hate to suggest it; Go Ultramarines. Purely because all the game seems to use them as the base-line to compare from.
However, Orks do offer plenty of stupid-fun. And converting your own Orkish vehicles is great. They don't need to look like they actually fit together. Since Orkish Tech seems to be held together by shouting and threats. They don't need to make sense.
But if you want an army with a tonne of Flexibility (Per Model), then Tau, packing lots of Crisis Suits, is the way to go.
Table-top is expensive, money-wise. So it's good to know that your Crisis suit could be equipped to deal with a different combat role, game to game.
Give them a pair of Burst Cannons- Anti Mass Infantry.
Pair of Plasma Rifles- Anti Heavy Infantry.
Pair of Fusion Blasters, Deep Striking behind tanks- Dead Tanks.
And the Tau Units which used to suck, now have uses. Vespids now have armour that can actually save them. And Kroot have Sniper Ammo in their rifles. (Which lets them MAYBE ignore armour.)