We have a lot of tools and sites that look at relative profitability of coins, and some that list things like 'market cap' but the metric I think is actually of value in determine which coins are on the top of the alt pile and to what degree they may be becoming rivals to BTC is to look at total hourly mining revenue. This is simply coins per block * blocks per hour * exchange rate to BTC (or) exchange rate dollars. For a relative comparison to BTC it's simpler to just use the exchange rate to BTC.
Now this analysis excludes the fact that a chain may be running faster then target (a common occurrence with people coin-jumping on profitability bubbles). I've included as broad a range of coins as possible, the 'Other' group here includes MinCoin, IXCoin, DeVCoin and ByteCoin and they sum to a paltry 2/3rd of a BTC per hour. For FRC I've summed the portion for miners and for foundation, the former is about 1/3rd of the total and the latter 2/3rds.
As you can see BTC is clearly dominant with right around 2/3rds of the total. Most of the remainder is composed of Scyrpt coins with LTC being the largest. The two flash in the pan coins FC and CNC (pump and dumps in my many other peoples opinions) combined come near to the revenue of LTC, the fact I've ignored these coins actual hash rates means they may be considerably higher, which simply goes to show how absurd their valuations are when they have no means to be spent outside of exchanges.
The remaining coins are SHA and are considered 'legit' by most folks having had some kind of technical improvement and are of similar size, PPC, TRC and FRC. FRC is currently running at around x4 target rates btw it would be producing around 18 BTCs in revenue per hour if that was accounted for, it's likely temporary though. Lastly we have NMC which is a merge-mined coin, its revenue is essentially in the BTC pie slice in a sense but not all BTC miners mine it. The tiny Other consists of numerous dead or dying coins, again if their hash rates have them far below target might make them even smaller but you can clearly see what a dead coin looks like.