what is "good" to u? what are u using it for? which flash can meet your needs in the most cost effective way? which one is the most user friendly? all these are important questions to ask yourself or u may end up wasting money.
Metz flash are good but some pple find the controls awkward because its an LED screen menu with very few buttons. but in terms of power, it is the brightest.
pentax flash have more buttons so it could be faster to change settings or switch to manual mode.
some users are happy with yongnuo manual flash - cheap and good - no pttl, but not necessarily needed if u know what u're doing.
i've been using the metz 48 for a long time - its basic and suits my limited needs when i need to chase my baby around. for bugs, i use an AF160 ring-flash... not likely that u'll find that to be a "good" flash to use even though i like to use it for certain specific conditions.
do some research. think about what u need. browse the specs online. download the instruction manuals. find out more before u go out and buy.