A plastic sheet - rigged properly, will keep you dry in the most torrential of rains. seriously.

If you are used to your tent leaking if it's not a "mountaineering tent" then your tent you are thinking of was probably a cheap POS bought at wal-mart or crappy tire, and then not waterproofed or seam sealed.

look back at some of the suggestions made here for 3 season use, and consider one of the
reputable ones. I've got plenty of tents - and tarps - that are not "mountaineering" stuff that have kept me dry in multi-day monsoon type downpours - as long as I did my part (seam sealed before taking them out the first time, didn't pitch them in a bowl that would fill up with water, etc.).. Keeping dry really isn't rocket science.