Your question has undoubtely been answered so I'll keep this brief:
My first thought was, Human because Swashbuckler makes an excellent jumping off point for other classes. Swashbuckler is already like a 1/2 Fighter 1/2 Thief, if you go from there and dual class to a Mage or Fighter or even a Cleric you're getting a beefy bonus to smacking stuff around.
If you're sure you want single class, I would suggest Elf. They get bonuses to bows and longswords and resistances to certain effects and have high Dex. Thieves don't get benefits from Con higher than 16, which negates some of the bonus of Half-Orcs and with 18 Strength you can read the book to get up to 19 bypassing the benefits of Fighter Ultra-Strength. Granted Half-Orc + Book is 20 but that's neither here nor there.
For a full saga run, Swashbuckler -> Mage dual is an absolutely solid choice. It's only a minor step down imo from the fighter/mage dual which is the OP build in the series. However it knocks out your thief requirements, allowing you to build a much more flexible party. Also, arguably a better solo build for the saga than fighter/mage.
Bonuses to thief skills by race, assuming maximum starting dexterity:
Race Dex PP OL FT MS HiS DI ST Tot
Human 18 25 25 10 20 15 0 5 100
H-Orc 18 25 25 10 20 15 0 5 100
H-Elf 18 35 25 10 20 20 0 5 115
Elf 19 35 25 15 30 30 0 10 145
Dwarf 17 20 30 20 15 10 5 10 110
Gnome 18 25 30 20 25 20 10 10 140
H-ling 19 35 35 20 35 35 0 10 170
Basically, being anything other than human or half-orc is worth 15 points, being short is another 25, and each point of Dex is 30.
With a swashbuckler, stealth matters less than other thief types, and that takes a lot of luster off the halfling or elf numbers. As a power-gaming choice, I'd go dwarf for the saves, half-orc for the immediate 19 strength, or human with intent to dual. Note also that a pure thief's options for increasing strength past 18 in BG1 are limited - you can't equip the gauntlets of ogre strength, and you can't drink giant strength potions. That leaves only getting there naturally (possibly with tome aid), using the Big-Fisted Belt from Rasaad's quest, or using short-duration spell buffs.
That said, the swashbuckler I played all the way through the saga was a halfling. Stealth may be less important, but having enough to scout even without investing points is great. There's plenty of room for less than perfect choices in these games.
Comments
For a full saga run, Swashbuckler -> Mage dual is an absolutely solid choice. It's only a minor step down imo from the fighter/mage dual which is the OP build in the series. However it knocks out your thief requirements, allowing you to build a much more flexible party. Also, arguably a better solo build for the saga than fighter/mage.
With a swashbuckler, stealth matters less than other thief types, and that takes a lot of luster off the halfling or elf numbers. As a power-gaming choice, I'd go dwarf for the saves, half-orc for the immediate 19 strength, or human with intent to dual. Note also that a pure thief's options for increasing strength past 18 in BG1 are limited - you can't equip the gauntlets of ogre strength, and you can't drink giant strength potions. That leaves only getting there naturally (possibly with tome aid), using the Big-Fisted Belt from Rasaad's quest, or using short-duration spell buffs.
That said, the swashbuckler I played all the way through the saga was a halfling. Stealth may be less important, but having enough to scout even without investing points is great. There's plenty of room for less than perfect choices in these games.