Fun to play? Orks. and as a "sub-army" grots. A typical 1500 point list for me with these runs about 120-140 infantry, and about 20 armoured units. It CAN be effective, but it can also fold like a cheap suit. But it's just funny to look at the blank stare of your opponent that first round with the muttering "What do i shoot at? so many things... "
for a grot army you do have to reach into Forge world a bit.
HQ: Big Mek
Elites: Grot tanks, (maybe a few other ork squads as support)
Troops: grots, nothing but grots.
Fast attack: Grot Mega tank, Dakka jets with the BS: 3
Heavy Support: Big Guns, Killer Kans, maybe some other vehicle support.