It depends on your business model and TLD firstly. Excluding the fact that many TLDs (ccTLDs pricisly speaking) don't give you few $ per day, I personally invest in generic keyword domains. These domains give small revenue at parking but they are very desired by companies - here you can really earn MONEY, instead of money from revenue.
For instance, in .PL ccTLD, the average value of 3-letter names raised in ~2 years (2007-2009) from few dollars up to 300-400 in domainer2domainer transactions, excluding values of trades towards companies. Here is the visible revenue, which has been growing all the time.
Less growth scale, but still impressive, there are in generics .DE, .CO.UK, .NL, .FR etc.
But the background of my statement is European domain market as you can suppose - I know that in US there are little bit different rules and for many there is the one king only - .com