Which candle is the tallest on the menorah?

The middle candle on the Menorah is called a Shamash - with the "a" being pronounced as in "father" and the stress being on the last syllable - [shaMASH].

However, it needn't be the middle candle - sometimes the shamash is at the end of the row and sometimes next to it.

Customary configurations would include:

xxxxSxxxx

Sxxxxxxxx or xxxxxxxxS

xxxxxxxx

......S

Some people have the custom of having 2 Shamash candles for each menorah.