This site is for the co-ordination of the Ubuntu project(s), and not support.
For Ubuntu Support options, please have a look at https://community.ubuntu.com/t/finding-help/712
Your question of “what is best” will be treated as off-topic on some, but why not ‘download’ and try one.
Lubuntu 18.04 uses LXDE (GTK+2 base) so maybe lighter if using GTK+ based apps, where as Lubuntu 18.10 & later are Qt based - so your end-use, or the applications you intend using will have a decent influence on which performs best for you (not just hardware), let alone the usual LTS versus non-LTS.
None of this however relates to Ubuntu project co-ordination, so don’t be surprised if a moderator closes this post (as off-topic).