I was looking today for some options to travel during the last week of March and I found many internet pages for buying tickets, so I was thinking which is the best you could recommend

I always use www.momondo.com

Searches in most airlines as well as other search portals (e.g. E-bookers).

I like this one.

try this kayak.com

Try www.mcflight.ch

my favourite. You can fly anywhere in the world, any time. It's really much more convenient dahling.

I tend to book direct with the airlines, but my advice is to keep clear of ebookers unless you 100% know that you're not going be ill, dead or in any other way unable to stick to the schedule. Changing routes or times is a no-no with these guys. I know that you pay for what you get, but go with the airline, at least they can be talked to. Just my $ 0.02.

I like this one: http://www.flugsupermarkt.com/ (german)