I personally like 8's for my SQ subbass and 15-18" woofers for SPL.

My next install will have either 2 15's or 1 18" (depending on if I get sponsored or not), along with at least 4 kilowatts of power. That's just for SPL.

SQ will be tweets, 6.5's mids, 6.5 midbaas, 8" subbass on 2 400 watt amps....brand depends on sponsorship ;)