What is the right way to send Alt + Tab in Ahk?

If you just want to switch back to the previous application, use Send, !{Esc}

$F1:: AltTab()
$F2:: AltTabMenu()

; AltTab-replacement for Windows 8:
AltTab(){
    list := ""
    WinGet, id, list
    Loop, %id%
    {
        this_ID := id%A_Index%
        IfWinActive, ahk_id %this_ID%
            continue    
        WinGetTitle, title, ahk_id %this_ID%
        If (title = "")
            continue
        If (!IsWindow(WinExist("ahk_id" . this_ID))) 
            continue
        WinActivate, ahk_id %this_ID%, ,2
            break
    }
}

; AltTabMenu-replacement for Windows 8:
AltTabMenu(){
    list := ""
    Menu, windows, Add
    Menu, windows, deleteAll
    WinGet, id, list
    Loop, %id%
    {
        this_ID := id%A_Index%
        WinGetTitle, title, ahk_id %this_ID%
        If (title = "")
            continue            
        If (!IsWindow(WinExist("ahk_id" . this_ID))) 
            continue
        Menu, windows, Add, %title%, ActivateTitle      
        WinGet, Path, ProcessPath, ahk_id %this_ID%
        Try 
            Menu, windows, Icon, %title%, %Path%,, 0
        Catch 
            Menu, windows, Icon, %title%, %A_WinDir%\System32\SHELL32.dll, 3, 0 
    }
    CoordMode, Mouse, Screen
    MouseMove, (0.4*A_ScreenWidth), (0.35*A_ScreenHeight)
    CoordMode, Menu, Screen
    Xm := (0.25*A_ScreenWidth)
    Ym := (0.25*A_ScreenHeight)
    Menu, windows, Show, %Xm%, %Ym%
}

ActivateTitle:
    SetTitleMatchMode 3
    WinActivate, %A_ThisMenuItem%
return

;-----------------------------------------------------------------
; Check whether the target window is activation target
;-----------------------------------------------------------------
IsWindow(hWnd){
    WinGet, dwStyle, Style, ahk_id %hWnd%
    if ((dwStyle&0x08000000) || !(dwStyle&0x10000000)) {
        return false
    }
    WinGet, dwExStyle, ExStyle, ahk_id %hWnd%
    if (dwExStyle & 0x00000080) {
        return false
    }
    WinGetClass, szClass, ahk_id %hWnd%
    if (szClass = "TApplication") {
        return false
    }
    return true
}

EDIT (suggested by the user Ooker):

The script pops up a menu for you to choose.

This is what it looks like:

You shouldn't manually send alt+tab as it is a special windows command, rather use the AltTab commands that do that for you.

AltTabMenu opens the tab menu and selects the program, whileAltTab, ShiftAltTab navigate through it.

h::AltTabMenu  
n::AltTab
m::ShiftAltTab