Sony Playstation 1
Quite simply the best console to collect for, in my opinion. The PS1 offers a room-filling 1400-odd games (plus Platinum, regional, and double-pack variants, if you wish). There are stacks of AAA titles and loads of hidden gems. In 2010 I was able to pick up hundreds of games for £1 each at car boot sales; you might struggle to find any decent titles in shops, but there are PS1 games in every charity shop in your town.
Some PS1 titles are already hitting the £40+ level; inexpensive classics such as Vib Ribbon sell for £10-20 on eBay and are likely to see a decent increase in the future. There are a lot of sports titles and childrens games that will probably only ever sell for £1 each, but as a highlt sucessful console that remains in the minds of millions of gamers, the PS1 is probably worth collecting if you aim to see a good return on your money in the future. The current eBay price (as of 2010) runs at over £5000, but if you buy the bulk of your collection locally you can expect to cut this outlay to £3000, especially if you find a good few bargains.
EBay prices for the PS1 vary greatly. With no massive interest in collecting PAL PS1 games at the moment, many rare gems go for 99p on eBay every day. A few titles e.g Castlevania: Symphony of the Night always sell for the same amount, give or take a few pounds (£100 is the norm for Castlevania) whilst rarer titles such as Blockids will go for 99p (or indeed not sell at all; my copy of Blockids had been relisted by the seller twice before I won it for 99p. Add in the fact that both games are available on the PSN for less than a fiver each, and the respective selling prices at auction become even odder. Yes, Catlevania is an amazing game whilst Blockids is just a fun five minutes, but normally in game collecting rare games that are rubbish are worth far more than good games that sold in large numbers. This seems to suggest that the future PS1 collectors market will see a massive rise in the rarer titles such as Blockids. As more people look to amass a complete PAL collection, many of these 99p games will surely be worth £30+… so pick them up while you can!