I've played a few boards but I'm looking for the more complex ones such as invention where you have the tech points. I want a challenge to make a hard game requiring a lot of strategy

you can filter for "advanced" boards on the board list.  Go to grid mode, then unclick the green and yellow lights.  Or maybe just unclick green and see what that leaves you.

You can check out my new board, Arena, it might be up your alley. There's an open game here:

http://www.wargear.net/games/join/525194

I'm a big fan of complex boards, so feel free to invite me to any that look interesting to you.

Unfortunately, there is not much in the way of 'tech tree' type games. I think the only other released board is simulgear so has a very different feel (http://www.wargear.net/boards/view/Axis+V+Allies+Tech).

psilofyr was working on a version of Risk of Thrones that had a tech tree element, but I don't think it ever got released. You could probably ask him to put in it in beta if you wanted to play some.

Some of my favorite more complicated maps for multiple players are:

• http://www.wargear.net/boards/view/Gates+of+Hell
• http://www.wargear.net/boards/view/Renaissance+Wars
• http://www.wargear.net/boards/view/Balance
• http://www.wargear.net/boards/view/Total+WarGear+II

For 2 player maps with lots of strategy try:

• http://www.wargear.net/boards/view/Iwo+Jima
• http://www.wargear.net/boards/view/Battle+of+Bladensburg
• http://www.wargear.net/boards/view/Battle+of+Waterloo

For a less complicated map that still has a little bit of the feel of Invention (because of the capitals and artillery) try: http://www.wargear.net/boards/view/Capital+Crusade

I might also suggest War of the Roses (or its big brother, Renaissance Wars) and Resource as intermediate boards that still have an epic strategy feel to them. None of these boards have a tech tree, per se, but they all have a more complex bonus generation system, and both Ren Wars and War of the Roses have a unique win condition based on reaching a variable threshold of resource control to claim the victory.

Kjeld wrote:

I might also suggest War of the Roses (or its big brother, Renaissance Wars) and Resource as intermediate boards that still have an epic strategy feel to them. None of these boards have a tech tree, per se, but they all have a more complex bonus generation system, and both Ren Wars and War of the Roses have a unique win condition based on reaching a variable threshold of resource control to claim the victory.

+1

A couple of my favorites with heavy strategic choice are:

Of Kings and Men

Seven Kingdoms

King of the Mountain 

Axis and Allies (simulgear!)

Definitely need to read the rules of the board to understand what's going on with these, and they offer multiple ways to approach the game, depending what your opponents are doing. 

Likewise, big boards like Europe 1560, HexGear, and Pipeline may be interesting -- with a medium number of players (7 or 8) there is enough elbow room that you end up playing the players as much as playing the board.  The boards themselves are pretty straightforward -- no major gimmicks.