Its been a long time since I bought one, but Manfrotto and Slik were always the top dogs - there may well be more on the market now
I have a 'Slik' as. the Manfrottos were always very heavy and I needed one I could carry easily

I would suggest you do as much research via mags/net/word of mouth as possible and then when you go to buy check it out thoroughly before handing over your money!
You will need one with a fully rotating pan and tilt head, and preferably a good central extension in addition to leg extensions - take it up to full height and check how solid it feels when its fully extended. a good one should stand firm, check that its got rubber feet as well as little spikes for solidity on any ground surface - thats about all I can tell you atm ... hope it helps.
Some of the other guys may well have makes they can recommend