From what I have seen on this site (as it is my main PS3 info/hacking source), it seems that Kmeaw and Rebug are the 2 most popular and stable. I have personally had zero issues with my kmeaw 3.55 CFW with MultiMan. Unless you need some special feature that one CFW offers that the others don't. Just make sure to do a system backup before installing any of them (just to be safe).
Strongly suggest going with Kmeaw 3.55 CFW first. Simply because its so "plug and play"-simple. Install it and then multiman takes about 10 minuts tops and most people will probably never need any more (well, today .. who knows what future brings). If you are not alone on your ps3 you might wanna install a "higher firmware spoofer" to protect the cfw .. or you might come home one day and you kid has accidentally overwritten your 3.55 cfw using a game the kid borrowed (oh! this happens a lot).
Rebug on the other hand is fairly technical and you need to update it (from the right folder) etc and probably turn off and on a couple of settings .. all good if you want a more techy and feature cfw .. but probably not good for the first install. Rebug is more for the techy programmer people i guess.
Some people may have troubles with Kmeaw but just as many have troubles with Rebug (all those updates arent just new features you know ).
Anyways, the good thing is that you can always alternate between the Kmeaw, Rebug and the official 3.55 firmware as much as you like.