The best coverage can be found for persons under 70 years of age. The highest amount for this type of "acute onset" coverage can be found in the Patriot America Plus plan which can be purchased with a maximum of $50,000, $100,000, or $500,000. The coverage for acute onset of pre-existing conditions is covered as any other condition and up to the maximum of the plan. Other policies offer lower levels of coverage for pre-existing conditions. Liaison Economy plan offers up to $15,000 maximum for the pre-existing condition regardless of the policy maximum for non-US citizens. Liaison Economy covers US citizens (traveling outside the US) with a primary insurance plan up to the maximum of the plan, however. Atlas Premium covers up to overall maximum limit and Liaison Choice up to $30,000 for non-US citizens.
Note that all the above are excellent comprehensive plans. However, if you would rather have a fixed plan (for an example of the difference, visit our fixed vs comprehensive details), some of those also offer coverage for pre-existing conditions. The best coverage in this category for pre-existing coverage can be found in any of the Inbound plans. They cover an acute onset of a pre-existing condition like any other condition. Note that with a fixed plan, the higher the maximum of the plan, the higher the individual caps on coverage, so I recommend getting as high a maximum as you can to maximize your coverage. It will still be cheaper than the cost of a comprehensive plan.
Note that once a person hits age 70, the pre-existing condition benefit either goes away completely or is significantly reduced (to $2,000 or so only). So even though you may be able to pay 69 year old prices if a traveler hits his 70th birthday while on vacation, this benefit will no longer apply from that date forward. So, check the next age category if you are hovering at the 69/70 year boundary.