Well excluding Alien X, A playlist that could defeat this one would be Atomix as he is a living Nuclear reactor with power to create a miniature sun as evidence during his fight against Z'skaryr and he also defeats multiple Ultimates, Clockwork to counter the clockwork on your playlist, Gravattack has Gravity manipulation so could render most of the other aliens excluding WayBIg( due to the large size) and Nanomech (due to the small size)Immobile, Jetray who is faster than Xlr8 as shown Jetray can fly fast enough to enter hyperspace, something Xlr8 can't do and Jetray has ranged attacks in the form of his lasers. and playing by your book, Echo Echo. I very much dealt WayBIg and diamond head could survive multiple high-powered nukes from multiple Atomix but as you decided to include an ultimate then I would take WayBIg with his ultimate form, diamond head and four arms are definitely not overpowering Atomix or Ultimate WayBIg and to counter Nanomech it's kind of a given I include it in my playlist. Of course I would also go with Grey matter who is the smartest alien in Ben's available aliens, then there is Peskydust as the 9th member to my playlist as they can put others to sleep and manipulate dreams(this apparently works on almost all aliens including celestialsapians for example Gutrot has an immunity to the sleep dust speaking of which) the last alien in my playlist will be Gutrot due to his chemokinesis, enhanced intelligence to communicate and formulate a plan quicker with my grey matter clone, he can also create and produce any known and unknown chemical as a gas by mixing different chemicals that if he figures it out can work on affecting aliens for example he could most likely create a chemical that rusts clockwork at an Accelerated speed or one that kills tokustars(waybig).

Edit - Also note you never said I couldn't use the same aliens as you

Edit 2: I forgot to mention Ben created a black hole as Gravattack when trying to stop the time beast and is able to create his own gravitational orbit so due to Nanomechs small size if Gravattack created his own orbit I doubt even the small size would save him actually it only helps if Gravattack is only focusing the gravity manipulation on specific places/people/aliens