Stay away from any clans that have provinces separated from each other, like Takeda. I remember the first time I played STW, and chose Takeda (I had recently watched the film Kagemusha) - I was defeated easily. Next game, I chose Hojo (and also set the difficulty level to easy), and took over the map.

So, I say Hojo is the best clan to start with. Rich provinces, and the reduced prices of castles make development far easier. Although you start off in (arguably) the hardest part of the map, surrounded by two clans (including the very aggressive Takeda clan who want your rich lands), you can easily defend your provinces and develop your army at the same time.

I think that the Shimazu clan are harder for a beginner. You should only play as Imagawa after you have beaten the game a couple of times - I find them the most enjoyable, but the hardest.