asian books are best, even when they are strongest in soccer, they offered a lot from USA
12bet was great from the beginning of the month, but after few months they realized that this is not the way to make money, i was taking away max payouts every day
now the best from asia is probably 188bet or sbobet, but i have not tested them deeply and i lost there all - so i can not say about payouts and i am not sure if they take US customers
decent are UK bookies - ladbrokes,
Bet365, WH so so, but they limit you fast, same is Bwin, Sportingbet, Unibet - wide range, high juice, limits come fast
pinnacle is universal low juice, and betfair of course must have for soccer and tennis
even UK bookies start now with live betting more and more - bodog has many games live, but the bets suck, since they have /had/ just few live props and i am not fan of them, but greek offers nationally televised foots and now even cris have both NFL and NCAA, limits for NCAA suck though... and of course they have MLB live; wsex is good, but only if they pay and if you are familiar with the horrible share system, but lines start as -108/-108 which is not bad
overall, bookies targeted for USA are weakest with live betting, but i bet we will se change soon