There are a few things to consider...
The Izula or Izula II is the traditional starting point. They are a great price point, and the knife's size belies its ability. It is a heck of a knife.
Then there is the HEST. I carry one of these every day, and they are a great blade. Rowen/ESEE will not be producing these any longer, so the current stock at retailers is it. If you think you might want one of these, by all means, buy one now. If you decide you do not like it, you should have no trouble getting rid of it for what you have in it.
The ESEE 4 is probably their best "all around" knife. Good size, great sheath. It s a very popular knife.
The ESEE 6 is like the 4, but longer.
The ESEE 5 is a beefy, tank of a knife. Some consider it a sharpened prybar. I think it would make a great TEOTWAWKI knife, but the thickness, weight, and saber grind keep it from being as versatile as some of the others in the lineup.
The Junglas is a beast. I keep one in my truck bag in case I ever need to chop through a tree that is blocking my way. Seriously.
You really cannot go wrong with an ESEE knife. Jeff and Mike are some of the best guys in the business to deal with as well. There is more info than you could ever want over in the ESEE subforum.
Hope this helps a little.