For the best sex it's important to have excellent cardiovascular fitness, strong and mobile hips, core strength and body control, and of course specific endurance in the muscles of the pelvic floor.
Cardiovascular fitness enables you to sustain sex both for long periods of time and for short but taxing sessions. Vascular fitness in general is important for blood flow to the genitals in both sexes, having the potential to increase the hardness and frequency of erections in men and the efficiency of vaginal lubrication as well as the facility of orgasm in women. Declining quality of erection in men can, in fact, be a marker for vascular dysfunction, whereas exercise, and aerobic exercise in particular, has been linked to improvements in erectile function (and reductions in rates of erectile dysfunction), endothelial cell function, and production of vascular nitric oxide  - the best sex will be difficult to achieve without cardio.
The sexual importance of hip strength, mobility and endurance is similarly not hard to understand. Good hip mobility will allow you to enter into a wider array of sexual positions (kama sutra look out!), and silly as it sounds, reduce the risk of injury. With immobile hips, your body is likely to use the lumbar spine to compensate for a lack of motion in the hips . This is problematic for a number of reasons: the spinal flexors and extensors are much weaker, and repeated lumbar rotation and end-range flexion and extension imposes a non-trivial risk to the intervertebral discs as well as the facet joints of the vertebrae . It is much better to achieve range of motion and power in the far stronger hips.
While the importance of core strength and stability varies based on position, it is generally necessary for transmitting force from the upper body to the lower body . For our purposes here, think embracing your partner above while moving your hips below. A strong core with and stamina will also help you keep your balance and sustain movement in certain positions, e.g. when picking up, or being picked up by a partner, or being in the top position.
While there's nothing sexy sounding about the bulbospongiosus or pubococcygeus muscles, they are foundational to the pelvic floor, and there are few more important muscles for sexual health and performance for both sexes. For men they contribute to erection and ejaculation, while for women they help to close the vagina and contribute to clitoral erection. They contribute to the feelings of orgasm in both men and women. For optimal sexual performance you'll want to develop both muscles, and you'll need to train them for both strength and endurance. Prepare to meet your friend, the kegel. A simple, but highly effective exercise for training the pelvic floor, kegel and pelvic-floor exercise regimens have been show to reduce premature ejaculation in men, , reduce rates of erectile dysfunction in men , improve both orgasm and arousal response in women , improve postpartum sexual function in women  , and add address urinary incontinence in both sexes . The value of kegel exercises and pelvic floor fitness for the best sex cannot be overstated, in some cases exceeding the clinical outcomes of even the most impressive medications. Just don't do your resisted kegels in the gym.
Finally, do note that we haven't touched the mental and emotional aspects of sex and orgasm, or sex appeal more broadly. These are undeniably important, but beyond the scope of simple physical fitness. That said, there are few superior methods of gaining confidence in a task than by training for it.
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In Best Sex - Sexual Fitness 1.5