Which property group colour gives the best ROI , undeveloped or fully developed, in Monopoly?
My understanding is that the best three property groups are light blue (Angel Islington, Euston Road, Pentonville Road), orange (Bow Street, Marlborough Street, Vine Street), and dark blue (Park Lane, Mayfair). Light blue is good because the buildings are very cheap but give excellent returns. Orange is good because the likelihood of landing there is substantially higher, due to forced starting from Jail (gets you on a 6, 8 or 9). Finally, the dark blues are a good choice because eventually someone will draw the Advance to Mayfair card.

There is a much more detailed analysis here. The names are from the American set, but the calculations are the same.