It's LG-Philips or Samsung. Samsung works alone while LG works with Philips.
Dell seems to prefer LG-Philips LCD panels but Samsung clearly tried to reduce Samsung LCD panel prices to OEM makers greatly to get some market share from LG-Philips and others. Lots more companies have switched to the latest Samsung panels (from LG-Philips.) Better features for lower cost, but that doesn't automatically mean Samsung panels are better.
In addition, Taiwanese LCD panel makers offer even lower prices, but each of them has much smaller production capacity compared to LG and Samsung so supply stream's not as good. In the CPU industry for PC, AMD sells cheaper than Intel but HP still buys CPUs from Intel because Intel has larger capacity and so can meet demand more easily and Intel has more room for price reduction.
Sometimes even the same LCD model uses more than one type of LCD panel and it's likely that the resellers and even the customer support people at the manufacturers may not know which panel each monitor on the market is based on, but for the same price, they use panels of similar specifications.
19-inch LCD monitors for 2x more than 17-inch LCD monitors is not worth investment I think. Panel price of 19-inch LCDs has dropped to nearly the level of 17-inch panels, perhaps around US$100. They say panel prices change daily depending on supply and demand situations on the market just like DRAM prices so even if you ask which panel model costs how much today, you have to ask again 10 days later if you plan to purchase then.