So, it is not a wise choice to aim for UM rangers just for its small increment of status (except you have unlimited amount of rubies..lol). Therefore, I will share on my opinion on which ranger should be given priority, so that we can use our Ultra Leonard *8 wisely.
IMHO, There are three main criteria that must be met to classify UM ranger as ‘worthy one’. Those criteria are:
Having skill that is level-multiplier
Example: Creator cony has skill that is based on level multiplier (duration = 0.1 second * ranger’s level). Therefore, if our ranger has lv 140, it will turn enemy into lollipop for 14 seconds. For UM ranger that has lv 240, the duration will be 24 seconds. Perhaps, in the future, there will be attack bonus that has level multiplier? (e.g: attack +% = 200% * ranger’s level), who knows? But I bet this will be future trend.
Not available on secret lab gacha or special stage
If the rangers are available in secret lab gacha or special stage, sooner or later we will get those rangers. So why bother to use our precious Ultra Leonard?
Sustainable skill and status for long-term.
Sustainable skill and status for long-term perhaps will be too subjective. Some will say it is sustainable, others will not. Ranger with buff skill tend to last longer. Based on my experience, rangers with very slow attack + move speed as well as non-buffer skill will be more likely to obsolete at a faster pace.