Here are some words beginning with the letter B that could be classed as 'nice'.
Beautiful
Bonny
Baby
Balmy
Note that these words do not have the same meaning as nice, just that they could be considered to represent something that's nice, or the notion of 'niceness'.
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Some nice words beginning with the letter B:BabeBeautifulBecomingBestBraveBrilliant
Bear starts with the letter B
Here are some possible words: backbitbacklitbailoutbaldestballastbanquetbaptistbarefitbaronetbassistbayonetbearcatbedfastbedpostbeleaptbelfastbenefitbequestbetwixtbigfootbiggestbiscuitblanketblatantblowoutboldestbombastboneletbonesetboniestbookletboomletborschtbouquetbowknotbowshotboxiestboycottbracketbravestbrisketbroughtbumboatbummestbuoyantburnoutbushtitbusiest
Bait, balk, bark, beam, bear and bend begin with the letter b. Boss, bunk, bush, buzz, boar and blue begin with the letter b.
A bumper sticker is sticky. It starts with the letter b.