Well you are quite limited to a few psychic types in generation 3, however they are all good in their own way. However I should say hat after Gardevoir, Starmie is the best psychic types but I will list them all
- [EDIT]Alakazam
- Gardevoir(Ralts)
- Medicham
- Grumpig
- Claydol
- Starmie
- Chimeco
- Wobbuffut
- [EDIT]Lunatone
- [EDIT]Solrock
- Metagross
Out of them all Gardevoir is used most often. This is because of it's high special attack, decent speed, and high special defense. It is bulky and has access to many powerful moves including thunderbolt and psychic. It is the easiest to obtain and the one that you will usually find most useful
Despite this, Medicham is a decent psychic type to use in generation 3. It has relative power and has access to very nice fighting and psychic moves. It is quite frail but it can also be used very decently. Well actually very good. This is because of Pure Power. This ability doubles it's attack stat meaning that it will always be hitting exceptionally hard
Grumpig is usually not used. This is because despite it's bulk and all, it is ugly and is quite slow. In-game you will appreciate it to have the ability to outpace things but this is not the case for this one
Claydol is a very decent Psychic type in-game. It can hit decently specially as well as physically and has access to a wide range of moves. However it is more of a supporting character then an attacking one so I would say you could perfectly use it as an attacker, however you might notice that it will not be hitting that extremely hard. Besides all this it can't be obtained quite early in-game
Starmie is quite a nice psychic type. This due to the fact that like most psychic types it has access to very powerful special attacking moves. Actually it should also be the best psychic type in-game as you can also utilize it as your personal surfer
Chimeco is not a psychic type you will be wanting...there is nothing else to say...
Wobbuffut is one interesting Pokemon, however it is extremely hard to find and althought it has access to both Mirror coat and Counter which will easily enable you to defeat the whole game, it is not worth spending the time looking for it
Metagross is by far the best psychic type in the entire game. It has a great attack as well as a great defense stat and can hit like a truck. However it can only be obtained after you have defeated the Elite 4
[EDIT]
Wtih many thanks to MM, I can tell you that Alakazam is most definitely one of the best psychic types in-game. It is extremely fast and it's special attack is something that every psychic type longs for. It is quite easy to get in-game just beware of those teleport Abra. Well actually it's quite hard since you will need to trade it between 2 games. However it's pre-evolution Kadabra is still a very good Pokemon
Solrock and Lunatone are both decent psychic types. However lthey are really outclassed byt the rest since their typing is not exactly a gift from heaven as almost every move that will hit you will most likely kill it as it is weak to water. ice, grass, ghost, dark..quite a lot of weaknesses and both of them are very weak in either special defense or defense respectively. Lunatone can't be obtained in Emerald so only Solrock is vaible
Girafarig is decent, not good not bad. It is relatively fast and sports a decent special attack as well as a decent attack stat. However it is not one that I would personally recommend. However it should be noted that it has a nice movepool
I would say Starmie, Alakazam and Medicham are your best options if not Gardevoir till the time you haven't obtained a Beldum. All three of them are very powerful and will serve exceptionally well. However if you still have a lot of patience and you don't mind training a level 5 Beldum till level 20 and then to level 45 after the Elite 4, I would recommend Metagross all the way
