Supposing a trading system (for some reasons) is designed for only fairly highly liquid markets: Practically, which are the most liquid markets? Are they quite different for private traders vs institutional firms? Is time-frame (such as intra-day or EOD) a major factor for the liquidity?
Currencies (spot & futures) & bond futures (US, Europe, Japan) Liquidity of your positions makes a big difference if you run a $500k account vs. a $5bln account.
also interest rate futures, Eurodollar, Euribor & Short Sterling are the most liquid futures markets in the world. Remember that spot currencies are mistakenly called the most "liquid" markets because people hear they do trillions each day - thats talking collectively with every bank, travel boutique and bucket shop added together around the world. All that global liquidity isnt available in one place like an exchange traded product - Joe Schmoe trading FX with a spot bucket shop with only a few million operating capital is not going to have access to trillions in liquidity - he deals with the price they make him, or not at all.
So far, I suppose that EUR/USD Spot, Eurodollar and Euribor futures would be the three most liquid markets for private traders, as long as proper platforms/ brokers can be arranged.
As a 'private trader' it doesn't make much difference. To a $100k, $500k or $1m account ES or ESTX50 (during RTH) are just as liquid as Eurodollar or Bunds or EUR/USD.