When none of the opening bids are suitable - what do you bid?
Don't be afraid to downgrade the strength of a hand by 1 HCP when it is flawed for the call being made. Here, treat a (exactly) 20 HCP hand as just 19 and open 1D. 

Remember that it is very rare for a distributional opening 1 bid to get passed out. Partner needs to have a hand with no excuse to respond; RHO needs a hand with no excuse to reopen; and LHO needs a hand with no decent 5-card suit. Just because you have a strong hand doesn't force you to open above the 1-level. With any doubt at all, just open 1D in tempo - Hands opening with a NT bid should be good examples of the type, not bad ones. 

As I commented to steenbergh's answer:


  Modern treatment (and rules!) now allows the 2NT opening with a singleton K as well [as with a singleton A]. This is an expert call, however, so should only be engaged in if partner is not going to react inappropriately when revealed.