As top tier in all three programs, I would say Hilton and then IC group. Depends where you travel and what's available to you though. IC hotels are everywhere, even where Hilton properties aren't in some rural areas where I travel.

Advantage Hilton: upgrades (real upgrades), free good breakfasts (at some properties anyway), generally higher quality hotels (although USA and UK are more uneven); upgraded accommodations with free breakfast (ie benefits) on reward stays; double dip for airline miles and hotel points;

Advantage IC Group: if you read the IC forum on FT you will find out many promos that will earn you extra points in the year, help you requalify easily and get you a whack of points for redeeming how you like (much more than Hilton which seems to have the fewest promos); ubiquitous locations; redeem points in any chain and get rooms with no significant capacity controls

both have good quality low end offerings (Hampton and HI Express) that generally are not run down, have free high speed internet etc.

Disadvantage Hilton: few promos; crappy properties in USA; some capacity control issues (less so as Diamond)

Disadvantage IC Group: crappy Holiday Inns, especially in USA (much better overseas); no benefits on reward stays; poor or no breakfasts (though adequate) at many properties in North America; top tier only in non-IC hotels; harder to achieve top tier Royal Ambassador which gets great benefits at "Intercontinental" name hotels; uneven access to upgrades (more so than Hiltons in my experience)