I assume you mean a European package holiday, not anything more exotic.
I think the sweet spot tends to be around January/February/March if you are fairly specific about where and when you want to go. My experience is that early bookers pay a premium and late bookers clearly have a reduced selection- hotels may be sold out or may only have the most expensive tier of rooms available for sale. But if you don't care where you go and you're flexible on times (e.g. you can go midweek to midweek) then there are late deals to be had.
I find the best value is with Jet2. This isn't just about their pricing either; their airport service knocks spots off the service anyone else offers and they generally have on-site reps in resort. They also tend to fly to most of their destinations daily so you can save some money by, for instance, going for 6 nights but taking an early flight out and a late flight back, which gives you the thick end of 7 days in resort.
EasyJet Holidays can be competitive on price but you don't get the same service, either at the airport or in the resort.
Living in the Isle of Man we have the added difficulty of getting on and off the island. So, although they don't offer the keenest prices and don't offer much service in the resort, I actually find that British Airways Holidays is a very good compromise for my needs, as BA offer through tickets from the island. This transfers the risk of misconnection from me to them and saves the hassle of going the night before and getting a hotel in Manchester or Liverpool at both ends of the holiday. And I get the Avios.
For long haul I used to like Virgin Holidays but they've retrenched and now really only offer holidays which are flown by Virgin Atlantic; unless you're going to the US or Jamaica they're pretty useless now. I've had good experiences with Kuoni but it's fair to say you pay for it; I've never personally used Trailfinders but I know many who have and they're always spoken fondly of. But again, service costs.