Hey Rhian
Whilst I'll always respect Graham's opinion, I have to disagree as I think Sharpies are the best.
Yes, the ink is permanent but they have a huge array of colours (do the Maxi Flo?) and in my experience they are not only great pens with deep, solid, vibrant colour but they seem to generally be the the 'weapon of choice' of the discerning balloon artist!
There are different styles/sizes of Sharpies around. You'll find many outlets that sell not only individual Sharpies but complete 'sleeves' of packs of all different colours. I'd say you're probably looking for a fine to medium tip as a general pen, but it's really what best suits you and what you're doing. As Graham says though, there are many threads here about pens and to be fair even some of the 'fat' Sharpies are not suitable for balloons (they cause the latex to pop) so be warned and have a look through the forum/try them out/form your own opinion before spending too much cash...
Hope it helps
--Neil