LXsession uses the window manager defined in ~/.config/lxsession/LXDE/desktop.conf (Openbox by default). If this file does not exist, it searches in /etc/xdg/lxsession/LXDE/desktop.conf instead.
Replace openbox-lxde in either file with a window manager of your choice:
For metacity:
window_manager=metacity
For compiz:
window_manager=compiz
Alternatively use WM --replace as defined in #Autostart, where WM is the name of the window manager executable being started. This means that openbox will be started first on each login and will then immediately be replaced. Note that Openbox and LXDE do not share the same rc.xml and keyboard shortcuts may differ. See xbindkeys.