VBA 1004 Error on Loop execution of Macro - vba

Question

Can anybody give me a sense of why I'd be receiving a 1004 error on the following code?
If it's not clear, I'm trying to loop all sheets that are not my named sheet and try to select a particular range and copy and paste it to the compiled "Quant Sheet"
Dim ws As Worksheet
Dim x As Integer
Dim y As Integer
Dim a As Integer
Dim b As Integer
Set ws = Worksheets("Quant Sheet")
x = 1
y = 3
a = 3
b = 2
Worksheets("Quant Sheet").Activate
For Each ws In ActiveWorkbook.Worksheets
If (ws.Name <> "Quant Sheet") Then
ws.Range("A3").Select
Selection.Copy
Sheets("Quant Sheet").Select
Cells(y, 1).Select
ActiveSheet.Paste
y = y + 1
End If
Next ws

Answer 1

You set WS as Worksheets("Quant Sheet") but then use that same variable ws to use in your loop. That may be causing the issue.
Try this:
Dim ws As Worksheet, mainWS As Worksheet
Dim x As Integer, y As Integer, a As Integer, b As Integer
Set mainWS = Worksheets("Quant Sheet")
x = 1
y = 3
a = 3
b = 2
For Each ws In ActiveWorkbook.Worksheets
If (ws.Name <> "Quant Sheet") Then
ws.Range("A3").Copy Destination:=mainWS.Cells(y, 1)
y = y + 1
End If
Next ws
Mainly, you want to avoid using .Select/.Activate to make sure you work more directly with the data.
Edit: FYI you can likely further make this more dynamic by not using something like y=y+1 and instead use offset, or a lastRow variable, but that's personal preference as it'll accomplish the same thing. (I'm also assuming the x, a, and b variables are used elsewhere in your macro...

Answer 2

As was already stated, you can't .Select a cell on a worksheet you haven't called .Activate on first - that would fix the problem, but leave you with frail & slow .Select and .Activate calls everywhere. Instead, iterate the Worksheets collection with a For Each loop, so you get a Worksheet object to work with each iteration:
Sub test()
Dim quantSheet As Worksheet, tempSheet as Worksheet
Dim i As Integer
Set quantSheet = ThisWorkbook.Worksheets("Quant Sheet")
i = 3
For Each tempSheet In ThisWorkbook.Worksheets
If tempSheet.Name <> quantSheet.Name Then
quantSheet.Cells(i, 1).Value = tempSheet.Range("A3").Value
i = i + 1
End If
Next tempSheet
End Sub
Further to the good answers and comments already provided, you can neaten up your code a lot.
Indentation is key. You can avoid loads of errors just by sticking to simple indentation
Remove of all those unused variables (unless you're using them later and haven't shown us!)
Rather than copying and pasting, set your values directly using .Value. It's quicker and better
Avoid Select and Activate as much as possible, as has already been pointed out. That includes ActiveSheet and ActiveWorkbook
Give your variables good, meaningful names and your code will almost read like a geeky VBA novel. That way you'll always know what's going on.
Post your working code on Code Review Stack Exchange for a full-blown peer review.