For me I think my favourite is Iwasaki's 15k. They say 15k but it's really more of a "true 10k". Ie., they're white, if you like a blue sheen you'd still need something supplementally but other than that these bulbs are great. The 175W I have outshines the 250W XM 10k's I have on another tank. AFAIK they are only available in mogul 175W and DE 250W. Would be nice to have more choice but I guess we get what we get. They're Iwasaki so they last forever before shifting (if ever) and the PAR is right up there (in case you care about PAR). Lastly, the 175's are probe start so they run perfectly on $30 M57 ballasts.
(Which is what I have .. nice to not have to worry about replacing the ballasts with pulse-starts one day.)