Star Walk 2 is very nice, with a rich set of capabilities and is very polished. Some great 3D rendering of objects And only $2.99. Sky Guide is $2.99 like Star Walk 2. Provides more local textual data as opposed to Star Walk taking you to on-line references (but it also has embedded links). Doesn't have the nice rendering of Star Walk.
Redshift is also very good but pricier at $9.99. Neat fly-to capabilities with good graphics.
Luminos - excellent but pricier yet at $14.99. Again, neat fly-to capabilities with good graphics. A lot of support for serious use with astro viewing gear.
I have all four of these and I'd say that for general usage that Star Walk 2 is the best because of capabilities and low price.