There is no single Cantonese word that is equivalent to "always" in all situations. Depending on the tense, you have to use different words as follows:
1. 永遠(wing5 jyun5)
I will always believe you.
我永遠相信你。(ngo5 wing5 jyun5 soeng1 seon3 nei5)
2. 成日(seng4 jat6)
He is always playing with his mobile phone.
佢成日玩手機。(keoi5 seng4 jat6 waan2 sau2 gei1)
* "成日" often but not always implies that you are not satisfied with the fact.
3. 一直都(jat1 zik6 dou1)
I have always wanted to be a doctor.
我一直都想做醫生。(ngo5 jat1 zik6 dou1 soeng2 zou6 ji1 saang1)
Actually there are many more.