i would like to know whats the best race in Oblivion to start an Assassin/Thief(using mysticism and alteration) character, race!

100 yard dash!

Seriously, there is no best in ES games. Each race has it own strengths and weaknesses. It all just depends on which of those you prefer. For example, since you mentioned using mysticism and alteration, an Altmer or Breton immediately leap to mind, a both those races start with bonuses to magicka. OTOH, a Bosmer starts with a +10 bonus to sneak, which is also handy for a thief/assassin type character.

Personally, I never look at these things when creating a character. I pick a race based upon whatever RP background I have in mind for the character. Plus I like to try every race at least once, so I spread around my racial choices just so I can see what they are all like. I found it can also be especially fun to play against the racial strengths/stereotypes, such as playing an Altmer barbarian, or an orc mage.

There is not best race. Your character is determined by your build.

As SubRosa has said, although races such as Bosmer and Breton have statistical advantages, at the end of the day it doesn't really matter. The question I'd ask is: which race looks the best?

But based purely on stats: why not try a Bosmer with The Mage or Apprentice as a birthsign?

Exactly. The word "best" is a bad, bad word, used in context of The Elder Scrolls games.

The OP is probably confused. These were not the answers he/she was hoping to hear.

Probably
Like others said,there is no perfect race for what you're looking for.Just pick the one you like best or think will fit the type you want to play.

Typical assassin/thief races are Argonian, Khajiit, Bosmer or Dark Elf.

But there is no "best". The above races would be lore friendly to assassin/thieves and start with some good stats. But eventually the starting stats don't matter, and other races can be great assassin thieves, and are in the game.

Yeah, best doesn't enter into it, because you can offset any early advantage within a few levels. And Sneak levels fast, and once that skill is at mid-to-high level, a small attribute difference doesn't matter. My best assassin character was a Redguard, who killed with a cutlass.