Something like: rogue 3 / Swashbuckler 3 / barbarian 1 / Swashbuclker +6 / Fighter 2 / Swashbuckler +5
Human from silverymoon with ewp elvish courtblade and education (from players guide to faerun) (as first level feats)
3 level feat is knowledge devotion (rogue with penetrating strike acf)
6 level feat is daring outlaw
7 level is barbarian (with spirit lion totem and whirling frenzy)
9 level mage bane
12 level wild cohort (AKA flanking buddy that gains HD, my personal favorite is a snake with martial study (cloak of deception), martial stance island of blades, darkstalker etc...)
fighter 1 is at 14 (martial study shadow hand (cloak of deception) grab the hit and run tactics fighter ACF)
fighter 2 is at 15 (martial stance assassin's stance and shadow blade)
18 level whatever, I personally don't like craven but I suppose that if you are going for damage you can take it or just power attack
This ends with a BAB of 19/14/9/4
But with knowledge devotion and education well... it can get a bonus +4 or +5 (if you have collector of stories it will probably be that high reliably)
You should have a high dex (even more bonus to hit, let's assume dex 26, because it is not that difficult to get, 30 is not really difficult either) and a high int (let's say 26 again).
knowledge checks 23+8+2 bonus = 30 if you roll a 1 you get +4 to attack and damage, if you roll a three or more you get a +5 to attack and damage (if you want you can reroll with collector of stories).
Total BAB (assuming an ordinary weapon)
let's get a magic weapon, something like a +1 keen elven courtblade of speed, collision and deadly precision. Add a friendly party caster with greater magic weapon or a tooth of leraje, or whatever to upp the bonus to +5
35/35/35/30/25/20 (speed grants extra attack, like haste, and whirling frenzy)
what about flanking? Well as long as you are by the snake you are flanking, that's a +4
39/39/39/34/29/24 of course you can always add morale bonus and stuff (with such a high int you can just buy use magic device or use psionic device cross class, note that since you have rogue levels UMD is caped at 23, but that is not really reliable) you get one more attack, for the sake of the argument the snake is not attacking (it is just hiding).
damage (let's assume I took craven, because why not)
1d10 (weapon damage) +5d6 (half SA damage) +5 (enhancement) +5 (collision) + 8 (dex) + 8 (int) + 8 (str, with a frenzy and a two handed weapon... easy) + 20 (craven)
Oh wait, applying SA is rather easy (flanking is rather easy)
1d10 + 11d6 + 5 + 5 + 8 + 8 + 8 + 20
And wait, if the enemy is flat footed you add your dex bonus again
1d10 + 11d6 + 5 + 5 + 8 + 8 + 8 + 8 + 20 = 1d10 + 11d6 + 62 (that is more damage that you were getting)
and you crit on a 15-20 you might crit for 2d10 + 11d6 + 124 + 2 STR damage
I haven't mentioned many magic items, but since this guy doesn't need magic to get pounce, only needs to enhance a weapon and can SA without needs of hiding and stuff he can pretty darn well get more goodies, and having such a good use magic device he can activate them easily. Right now bracers of murder (drow of the underdark seem great) but since custom magic item are on the table just add them bracers of the hunter (secrets of xen'drik), +1d6 SA and reroll SA dices that result in a 1 (and +2 initiative, +4 to hide). A rogue's vest is another +1d6. Grab a belt of battle or something. Or just buff your defenses. After that, well I suppose you can invest on those scrolls of greater heroism and dorjes...
for 1d10+13d6 (reroll ones) + 62
Init: 21=8(dex)+2(fighter)+2(bracers)+4 (wand of sign)+5 (wand of neverskitter) + (if you want you can actually buy an eager weapon)
That ends doing more damage and hitting more often, with one weapon, and I haven't gone over all the possible magic items, there are much better iterations.